The unsaturated, dark values are an important aspect of the design, as they make the scarlet flowers "pop" (appear by contrast relatively brighter and more saturated). All the normally bright or light valued colors, such as whites or yellows, have been darkened or muted toward grayish greens and blues, to emphasize this effect. The variety of paint impasto and brushstroke textures also contribute to the overall impact. A Color Map. To clarify our color analysis, we first require a framework that permits the hue and chroma, or hue and lightness, in any painting to be accurately described. This framework would also allow the colors in different paintings, or in a visual style, to be summarized and compared. My solution is a color map. This is made by measuring the hue and lightness of a large, systematic sample of single pixels from an electronic image of the painting, then plotting the location of these values inside a palette scheme. (Lightness is more accurately recorded than chroma in an electronic image.) Here's what I get for 250 pixels sampled from Gauguin's painting: color map of the gauguin painting 250 single pixel measurements of hue angle and lightness sampled from a large format digital image; lightness is shown as distance from center (black) to circumference (white); 12 tertiary hues shown for reference The main distortion that is introduced here is that the image content has been discarded. All the poetry of the original image has been chopped away: the bouquet has been shredded into compost. And this is the first insight into color design assumptions: most "color harmony" schemes adopt the Frankenstein premise that you can make a lively painting by stitching together bits and pieces of color. In fact very hard to make the leap from compost to a bouquet, from an abstract color scheme to a specific assignment of colors to objects in an image  one must also consider the value design, the style of representation, the size of the image, and so on. These are visibly important aspects of the Gauguin work, yet color design implies you must focus on the inherent qualities of colors in order to work out their "color harmony". A Palette Scheme. The natural next step is to the inherent quality of paints. The color map is useful because it outlines the color space or gamut of the painting. In any gamut, the most saturated, pure pigments or paints will appear at the extreme points or "corners" of the distribution of mixtures. So there is clearly a scarlet pigment in the palette, probably vermilion (mercuric sulfide, PR106); there is also a middle yellow (either cadmium or chrome yellow), a reddish blue (cobalt or ultramarine blue), and a dark green paint. (This can't be judged from an image; it could be a bluish green pigment such as viridian, a convenience mixture such as cadmium green, or a palette mixture of the blue and yellow paints.) Unsaturated pigments within the gamut are harder to identify, but are usually indicated by heavy clusters or lumps of color within the color map. There are three of these, and they suggest yellow ochre, burnt sienna and burnt umber were included. Finally, a warm white was necessary for the light valued table top, and a black paint was probably used as well. So Gauguin plausibly used at least nine separate paints on his canvas. Based on the conjectural paint selection derived from the gamut of the color map, we can summarize Gauguin's palette as an abstract or idealized palette scheme. a plausible palette scheme for the gauguin painting This example illustrates two important points: (1) the implementation of a color scheme depends on the available choices of pigments, dyes or colored materials (yarns, tiles) in the media that can be used to represent it; and (2) the use of a specific medium always defines a specific gamut, which produces unique limitations on the color relationships (saturation or purity of hue and value contrasts) between different hue categories. Two new, important distortions are added here. First, paints colors are treated as abstract hue categories rather than as specific kinds of materials. We are no longer thinking in terms of material colors  the unique colors produced by a specific color medium, but in terms of concept colors anchored with a generic color name  yellow, red, medium yellow, bluish red, and so on. It will be important later to distinguish concept and material colors: to do this, concept colors will be written in small capitals, as YELLOW or RED. Note that the concept colors used in color theory are typically hue labels that do not specify anything about the saturation, lightness or darkness of the color. The palette scheme cannot tell us if Gauguin used his paints as pure colors or "broke" them in various mixtures with each other. The second distortion is that the color proportions have been discarded. We have no way of knowing, from the palette scheme, how much of each paint color was used in the original painting, or even what are the proportions among these colors that would give the best overall effect. We have the color ingredients, but no instructions for how to combine them in an image. These omissions are potentially more serious than shredding up the image content to make a color map, because materials and proportions significantly affect our reaction to color, even when color appears in abstract textile designs or in the choice of wall, carpet and furniture colors in interior design. A Color Harmony. Now, how did Gauguin arrive at his specific selection of paints? The standard color theory answer is that he chose a color design for the painting based on color harmony and contrast, used this design to guide the choice of paints, and then applied these paints in ways that preserved the intended harmony and contrast while matching his intended visual design and the appearance of the still life he was painting. OK, so how did he get the color harmony? Here "color theorists" simply lay down the dogma that certain colors do or do not produce harmony or contrast; most justify these color relationships in terms of primary color relationships. In the outcome, the painter simply chooses from among a group of predefined color patterns or color schemes or hue harmonies  analogous, complementary, triadic, etc. The scheme is a kind of convention or recipe intended to ensure pleasing or effective color effects. the split complementary color harmony One might expect that these color recipes are fairly specific, but in fact "color theory" makes them completely abstract  analogous colors, complementary colors and so that are independent of which hues we are talking about. So long as you talk about these color relationships, it does not matter which specific hues they refer to. A perennial expression of this disregard for specific hue relationships is the circular color calculator, which shows the harmonious or contrasting color combinations for any specific key color or anchor color you might choose. The example below, the color systems dial devised by the workshop watercolorist Zoltan Szabo shows all the theoretically related colors for any categorical key color (red violet in the example) that the artist might choose.

zoltan szabo's color systems dial the key color (top) is red violet The general idea is that the painter can spin the dial to find appropriate combinations of colors for the specific key color under consideration. Note that the windows in the card, the relationships among abstract color categories, are the actual "color harmony": the hues that appear through the window are just interchangeable tokens. These color calculators always remind me of the circular slide rule wielded to hilarious effect by Dr. Strangelove. They are, incidentally, an idea first proposed by J.W. von Goethe  who imagined "a moveable diametrical index in the colorific circle" to identify complementary color pairs. But what happens if we apply this color dial logic and "spin the hues" of Gauguin's painting? The illustrations below shows what happens if we rotate all the color relationships on the "color calculator" by a single step toward magenta or toward green.

a "color systems dial" change in the gauguin color scheme all hues shifted by one tertiary hue step toward magenta (left)

or toward green (right) Eeek! Clearly, this is not right. By shifting the colors farther toward red, we've traded the potent contrast between scarlet and umber for the much weaker contrast between red and maroon, collapsing the color depth as a result. Alternately, by shifting the color emphasis toward green, we have created a sickly light and an unpleasant orangish yellow for the blossoms. Both images are less pleasing than the original. This is why the "color theory" suggestion that you "spin the colors" is typically not useful: the peculiar types of contrasts, color changes and color relationships that appear in one hue area of the hue circle often do not apply in other hues, or a different lightness or saturation. The "complementary" contrast between purple and green is almost restful, the contrast between orange and cerulean is cheerful, the contrast between yellow and ultramarine is stark  but the contrast between spectrum red and greenish blue is downright hostile. By reducing specific, unique color combinations to abstract, spinnable color relationships, "color theory" achieves perhaps the most remarkable feat of all: it discards color itself, and only considers contrasts or similarities. A Viewing Context. There is a final problem that rightfully should fall within the scope of "color theory", and that is the appropriate combination of colors in an environmental setting  for example, the best choice of context in which to display a painting, or the best choice of painting to complement a specific architectural space or that trendy couch you've bought at Roche Bobois. According to Kenneth Burchett, the traditional discussions of color harmony equate it with concepts such as order, configuration, interaction, similarity, association and area.  in other words, context. In the simple example below, we've bought a nice print of the Gauguin painting, but find that the context in which we display it affects our sense of its color harmony. gauguin's painting in two different rooms How do we decide which setting works better, and what reasons do we have for the decision? Presumably if we have a theory of color harmony that really works, it would be able to explain the harmony or clash between colors across object boundaries  between a painting and a wall, or a couch and a carpet. What is Color Harmony, Anyway? Which brings us to the most fundamental question of all: how do we define color harmony? In this page I argue that: Color harmony is the manipulation of lightness and chroma within a given selection of hues so that all colors contribute to an intended visual effect. OK ... so what visual effect? We must stipulate at least four criterion contexts that determine whether color relationships are satisfactory or not, and each context implies different considerations in color design:  Pattern: the effect of color combinations to preserve the overall clarity of a figure/ground pattern, or alternately to blend visually into a specific visual mixture or texture: the legible effect of the color choices.  Object: the effect of the color combinations to produce visual cohesion ("goes together") or contrast ("stands out") within a single designed or significant object, such as a book cover, vase, painting or the apparel worn by the person: the cohesive effect of the color choices in relation to the environment.  Representation: the effect of the color combinations to produce a pleasing overall effect within a representational image, such as a photograph or painting: the pictorial effect of the color choices.  Environment: the cumulative effect of all material colors and light colors visible in a place, setting or environment, natural or engineered, architectural or mechanical (such as the cabin of a car), which combine to create a visual mood, local atmosphere, task setting or spatial effect: the ambient effect of the color choices. These four criteria  PORE in acronym  do not lead to the same conclusions about color contrast. For example, here are the principal color contrasts in the lower left or upper right quadrants of the Gauguin painting, presented in a simple textile contrast pattern that displays the relative dominance, contrast and visual mixture among three colors. textile patterns using gauguin's color choices in area proportions 10:11:30 (right diagonal:left diagonal:background) These patterns reinforce what we conclude by looking at the painting: (1) the scarlet, umber and yellow gray in the lower left of the painting (diagram, top row) are used to produce a strong visual contrast between the blossoms and the rounded forms against their backgrounds, and this contrast depends primarily on the dark umber separating the dull yellow from the scarlet (left pattern); and (2) the green, purple and mauve in the upper right of the painting (diagram, bottom row) are used to reduce contrast, which pushes these colors into the background and provides a cool color contrast to the predominantly warm hues of the painting. However, if we evaluate the colors as textile designs, other issues become more important. What is the overall color of the design when seen from a distance  is it dark or light, green or brown, warm or cool? How strongly does one color dominate the other two? What is the dominant visual pattern that results from all three colors combined  left diagonal lines, right diagonal lines, a diagonal mesh, or a field of crosses? Which pattern and average color works best in the object, fabric or environment in which the colors are used? If we change the context in which color relationships are judged, we are likely to prefer different color combinations for different reasons. The optimal color combinations and our reasons for preferring them change together. Toward a Natural Color Harmony. What emerges as the basic issues in color harmony and color design? An important first step is a vocabulary to distinguish the different aspects of color we are talking about:  material color - the composition of light reflected from a surface or emitted by a light; always precisely defined by a spectrophotometric curve, but influenced by surface qualities of materials or the light absorbing qualities of light transmitting media.  visual color - the conscious sensation of color, viewed in a specific visual context under a specific level of illumination; always approximately defined by the colormaking attributes, plus luminance: this includes appearance color  visual colors induced by viewing context.  concept color - color treated in the abstract or as a "pure" color that cannot exist in materials, usually as a category of colors denoted by hue label (primary BLUE) or by some other generic attribute (all SHADOW COLORS, all TINTS). I argue that a useful and modern theory of color harmony and color design must include:  A compact summary of the necessary and sufficient principles of color perception, and including the effects of luminance and illuminant on color, stated as generalizations about harmony, contrast and dissonance;  The visual perceptual principles adapted as necessary to represent the capabilities and limitations of specific colored materials or lights  stained glass and architectural illumination systems; dyes and pigments used in paints and inks applied to paper, photographic emulsions, canvas, textiles, plastics and ceramic glazes; electronic phosphors and lights used in televisions and computer monitors;  The use, application, purpose or significance of the colored artifact;  The viewing context or situation in which the colored artifact is encountered. This page amplifies on these issues and concludes with an outline of my own "natural color harmony". Though incomplete, I believe it clearly illustrates the general direction in which any color theory must progress. Some interesting color management or color analysis tools are now available online. Kuler is available from the Adobe website. A fun online application to decompose electronic images into color swatches is available from the Colr.org website. Summary of Key Themes. Most traditional theories of color harmony prescribe the manipulation of two colormaking attributes while holding a third attribute constant; in this respect the hue mixture theories of Newton and Arnheim are primitive, despite their conceptual elegance. Looking back across three centuries of color exploration, we can glean the following twelve principles from the historical study of color harmony: 1. Color harmony is traditionally defined as (1) proportions or ratios between fundamental color qualities (such as "primary" colors); (2) straight line samples drawn within a color space, the lines being divided into scales of equal or geometric color intervals; or (3) consistent relative locations around the hue circle (in particular, analogous and complementary hues). 2. Colors harmonize when they are variations within the achromatic grayscale (achromatic harmony). 3. Colors harmonize if they are variations in lightness and chroma within a single hue (monochromatic harmony). 4. Colors harmonize if they are variations of constant saturation across lightness within a single hue (shadow harmony). 5. Colors harmonize more, the smaller the hue difference between them (analogous harmony). 6. Colors harmonize more, the smaller the chroma difference between them, regardless of lightness or hue (chromatic harmony). 7. Colors harmonize if they are the same lightness and chroma, regardless of hue (nuance harmony). 8. Colors harmonize more, the greater their average lightness, regardless of hue (brightness harmony). 9. Colors harmonize more, when there are moderate but not extreme lightness differences among them (lightness variety). 10. Colors within a perceptual color space assort best if they are selected to lie on a single plane oriented at any angle to the lightness and opponent hue dimensions (plane harmony). 11. Colors harmonize more, when lighter valued and/or more saturated colors occupy a smaller visual area than darker and duller colors, in proportions equivalent to the degree of color differences among them (color proportion harmony). 12. Colors harmonize less and contrast more the larger the hue difference between them (hue contrast). 13. Colors contrast most pleasingly if they are complementary hues (complementary contrast). click the image for a library of the OSA UCS color system

At the same time, we can identify five unresolved issues across theories of color harmony:  Material or Visual Color? - Should color harmony be defined in terms of material color mixtures (Arnheim, Birren, NCS), or visual color mixtures (Newton, Chevreul, OSA-UCS)? This is most significant when regulating chroma and lightness contrasts.  Similarity or Contrast? - Are colors harmonious because they blend well (Chevreul), or because they contrast effectively (Munsell)?  Subjective or Conceptual? - Should color harmony be defined according to subjective criteria (pleasant/unpleasant, attractive/unattractive) or formulaic, rational criteria (relative area, primary color combinations, colors within a scale, equality in lightness or chroma)?  Symmetrical or Irregular Geometry? - Should conceptual color harmonies be defined in terms of a colorant geometry (circle, sphere, triangle, cube, double cone; Chevreul, Arnheim, Birren, NCS) or should it be defined within a perceptual geometry (cylinder enclosing an irregularly shaped perceptual space; Munsell, Coloroid, OSA-UCS)?  Invariant or Contextual? - Should harmony be defined as an invariant quality of color combinations that is completely independent of the situation in which colors are viewed or the purposes for which the color is used (Rumford, Arnheim, Munsell)? Or should color harmony depend on the viewing context or functional use of color, so that colors that are harmonious in one situation may not be harmonious in another (Birren)? concepts in natural color harmony Now that we have in hand the common artistic language of traditional hue harmonies, and have given thoughtful consideration to seven previous theories of color harmony, we can approach the task of formlating a truly modern, practical and effective theory of color harmony and color design. For reasons that I'll make clear later, I call these principles a natural color harmony. This section introduces the fundamental concepts in natural color harmony, which are the concepts and terms that must underlie any accurate discussion of color. The next section develops these concepts as principles of natural color harmony, or a basic framework for understanding and applying color. A Definition of Color. To begin with, what do we mean when we talk about color? As explained in the page on the Basic Forms of Color: Color is a contextual interpretation of surfaces illuminated by light in space. We do not experience color as an abstract sensory quality but as an inherent quality of objects situated in the physical world. As I will illustrate below, color depends to a great extent on our implicit assumptions about the quantity and color of light in the environment, the direction light is coming from, the locations in three dimensional space of the objects that the light illuminates, the contrasts created in light shadowed or reflected by these objects, and the appearance of these object colors in comparison to a white surface. All these features are combined in what amounts to an automatic, unconscious and highly sophisticated interpretation of the real world context in which we see things. I emphasize the concept of "interpretation" to prepare the idea that color is not something that is "in" the light or "in" the surface of objects or even "in" the response of our L, M and S photoreceptors. Color is in the mind of the viewer. This frees us from thinking of colors as having fixed attributes, and requires that we see colors as mediating or clarifying the dynamic relationship between the physical environment and our conscious interpretation of it. Colors resemble ideas more than they resemble sensations; they are perceptions  ideas in the guise of sensations. Three Kinds of Color. Despite the previous definition, it is perfectly sensible to talk about a "color" of paint or a "color" that we imagine rather than see. One of the difficulties in talking about color is that color seems to exist in several different forms. Three of these are most important:  material color is the mixture of light wavelengths emitted by a light source, or transmitted by a filter or other semitransparent medium, or reflected from an opaque material such as paint, ink, dye, photographic paper or many kinds of material surfaces. This defines color very narrowly, as a physical stimulus that we examine without any consideration of the context around it. a reflectance curve showing the percentage of incident light (vertical scale) that is reflected at each wavelength (horizontal scale) within the visible spectrum (400 nm to 700 nm) Material color is exactly specified as a spectrophotometric curve (diagram, above), which can be measured in lights, filters, surfaces and colored substances such as pigments, inks or dyes. When measuring material color, the principal assumption is that the surface attributes of the material (including color unevenness, texture, gloss or mirrorlike reflectivity, iridescence, and translucency) do not significantly affect the spectrophotometric measurement.  visual color is a perception of a material color in a specific viewing context  usually the color of a physical surface viewed in a specific place under a specific intensity and color of illumination. Visual color literally does not exist outside individual consciousness. It is personal, immediate, and always embedded in a context. There is an enormous body of evidence to show that color experience is remarkably personal: it varies significantly across individuals, for a variety of reasons (genetics, age, experience). In addition, the same material color can appear as very different visual colors, depending on the light environment and surrounding colors where it is viewed. As a practical matter, then, the connection between a material color and visual color can be highly variable across individuals and viewing contexts. Although visual color is personal, it does not have to be private. We can fairly reliably communicate visual color to other people through a variety of color specification strategies. For these to be effective, five conditions must be met when a material color is visually examined: (1) distractions caused by the surface qualities of the material color (such as color irregularities, surface reflections or iridescence) are eliminated or minimized; (2) the color is illuminated to approximately daylight brightness; (3) the illumination is both "daylight white" and broadband (i.e., the light includes all visible wavelengths at roughly equal energy); (4) the color is displayed against a medium gray background; and (5) the viewer's eye is accustomed or adapted to the background color and intensity of the illumination. When these conditions are met, visual colors can be specified by mathematically translating the material color (the reflectance curve) into a color appearance specification on the three colormaking attributes (discussed below); or by finding the best visual match between the material color and the material color samples published as a standard color atlas; or by matching the material color to a color sample defined as a color "address" in a color reproduction system (such as the code "#336699" in the digital RGB color space or the formula "30-50-15-5" in the Pantone CYMK system). Note that these are not "different kinds" of color, but rather different ways to specify a standard visual color or a material color as the stimulus standard for the visual color.  conceptual color is color as an abstract concept, a sensory memory, a color label that calls to mind a visual or material color that is not present as a physical exemplar or as a visual perception. It is color defined primarily through language, memory, custom and habit. Conceptual colors can be communicated as single color words (auburn, chartreuse), compound color descriptions (brilliant dark blue), the average color of a variable environmental stimulus (sky blue, sea green, cherry red), a color in color theory (primary blue; "yellow and blue make green"), an imaginary color ("no paint can be a pure red") a metaphorical color ("a golden sunset"), and much more. Compared to both material colors and visual colors, conceptual colors are simplifications in three respects: (1) they are categorical colors that apply equally to many different kinds of material or visual color (blue can refer to eyes, skies, berries, plastics, flowers, textiles, ceramics, paints, stained glass, photographic emulsions, television screens and lakes); (2) they can refer to colors that are unknown to the person to whom the color is being described ("yellow is the color of my true love's hair"); (3) they disregard variation produced by individual differences in color sensitivity and viewing conditions (a lawn viewed at twilight is still called green); and (4) they assume that color descriptions mean approximately the same thing to all people. These simplifications make conceptual colors very useful in the general framework of language, but extremely troublesome as the basis to talk about color to any specific purpose. It is extremely helpful to keep in mind the differences between material, visual and conceptual colors when thinking about color across the many topics in color thoery. Three Visual Color Attributes. As introduced above, all visual colors can be minimally identified in terms of three basic attributes:  brightness/lightness. These are the color attributes directly related to the luminance of a color area, perceived either as an emitting light (that is, a surface that shines more brightly than a pure white) or as a reflecting surface, as described below.  hue. This is simply the "color" in color, the identifying chromatic content separate from variations in luminance. As explained below, hue is conventionally diagrammed as a hue circle. Hue can always be exactly defined as a single wavelength of spectral light, or (for violet hues) either as the mixture of single "red" and "blue" wavelengths or as the complementary "green" hue that is directly opposite on the hue circle.  hue purity. Hue purity is my term for the third dimension of color, which been defined many different ways across the past two centuries of color research  the point being that "hue purity" is not specific as to how this third attribute is defined. In terms of visual color, hue purity can be reduced to three distinct concepts:  colorfulness is the proportion of "pure hue" or pure chromatic reflectance (C) in a color, as a proportion of the sum of the pure chromatic content plus any white (W) reflectance and/or black (K) absorptance, when the color is viewed in isolation, without compensating for the illumination on the color. As a conceptual formula: colorfulness = C / (C+W+K) Colorfulness is equivalent to the "pure color" component in every colorant color space, which defines colors as a mixture of pure color pigment, white pigment and black pigment. It is therefore the quality of hue purity that is most analogous to material color (as explained below). We only perceive colorfulness if we compare the color to an imaginary "pure" color,  chroma is the relative amount of chromatic content absorbed by a color in comparison to the relative amount of luminance absorbed by the color.

The quality of colorfulness in surface colors, including paints and inks, perceptually increases with increasing illumination: all surface colors appear to darken or blacken under dim light, and even dull colors appear to increase their chromatic content in sunlight. To remove this sensitivity to illumination, chroma evaluates the color area by comparison with a "white" surface under identical illumination (diagram, right). In chroma, the standard used to define "relative" for both chromatic and luminous content is a pure white surface under the same illumination as the colored surface. The comparison is just the difference between the two relative quantities. As a conceptual formula: chroma c = ( RGB c / RGB w ) - ( L c / L w ) where the subscripts c and w refer to attributes of the colored and white surfaces, RGB represents the separate trichromatic color signals or stimulus measurements, and L is the luminance of the two surfaces. The logic of this concept is not difficult. A "white" surface is two things at once: it is a balanced signal among three different chromatic outputs, for example the L, M and S cones or the XYZ tristimulus values; and it is also a source of luminance. A colored surface is an imbalanced signal among three different chromatic outputs, but it is also a source of (relatively darkened) luminance. To define the chroma, we first define the proportion of luminance in the colored surface compared to the white surface, then we define the proportion of matching chromatic outputs between the two surfaces. If the chromatic proportion is either greater than or less than the luminance proportion, then the chromatic outputs are different from the chromatic outputs in a white (colorless) surface, meaning that chromatic content is present.  saturation is the chromatic content of a color judged (for lights) in relation to its own brightness, or (for surfaces) in relation to a gray or white of equal lightness under equal illumination. As a conceptual formula: saturation = chroma c / brightness c Saturation is entirely a perceptual quality, since both the chromatic content and brightness/lightness are visual color attributes. That is, we cannot use luminance to define saturation, only the perception of relative luminance as brightness or lightness. In particular, the saturation of an unfiltered light does not change if we make it brighter or dimmer, and the saturation of a surface color does not change if we increase or decrease illumination on it  for example, surfaces in sunlight and in shadow have the same saturation, even though the shadowed surface is much darker. Material colors can be described in terms of the three colormaking attributes by means of the measurement technology colorimetry. This requires (1) measurement of the emittance or reflectance profile by a spectrophotometer, which captures the intensity of light at each wavelength; and (2) the transformation of the measured wavelength intensities into the perceptual colormaking attributes by means of a mathematical model of color vision. comparison of

colorfulness and chroma (right) we perceive the "same" orange color when we take into account any differences in illumination

(= chroma or saturation);

(left) seen as an isolated color, the black content of the orange has changed

(= colorfulness)

By definition, conceptual colors are not perceived, so they do not in themselves have perceptual attributes. But the colormaking attributes can be appropriated to the logical or verbal definition of a conceptual color  as is done, for example, with the theoretical definition of optimal colors. Three Material Color Components. Watercolor painters, as a tribe, are fond of saying that "primary" colors cannot mix the entire range of visual colors because all colors of paint or ink are impure. But as explained here in more detail, the limited mixing range of material colorants has to do with the behavior of light as it interacts with color materials, not with their "impurity". All material colors comprise three different components of light:  scattering (W). When light strikes any object, some portion of it does not affect any change in the material itself. The light is simply reflected back into the environment from the surface boundary between the material and the air, in random directions that depend on microscopic variations in the surface. This is surface scattering of light, and it adds whiteness to the visual color of the material.  chromatic reflectance (C). A substantial portion of the incident light passes through the surface boundary and throws its energy into the molecules that compose the material. This elevates the electron energies within the molecules. These disrupted electrons promptly emit the light again at specific wavelength energies that will return them to their original energy state; these discrete wavelengths aggregate into the material's unique chromatic reflectance. Some materials produce greater chromatic reflectance than others, producing an increase in the apparent chroma or saturation of the color. And many synthetic red, orange and yellow pigments produce chromatic reflectance that is very close to the physically possible maximum hue purity.  infrared reflectance (K). The remainder of the light energy absorbed by the material is emitted by the electrons at much longer wavelengths, as heat. Some of this heat is held in the material, the rest is radiated back into the environment. However heat is invisible to the human eye  a proportion of light goes into the material, but nothing visible comes out  so the radiated heat adds a component of blackness to the visual color. This is the accurate statement of the "impurity" in material colors: All visual colors produced by light reflected from materials comprise a proportion of whiteness (W) caused by surface scattering, a proportion of pure chromatic reflectance (C), and a proportion of blackness (K) caused by the transformation of light into heat: visual color = W + C + K This formulation is too sweeping, as there are many materials that produce color by other means, such as refraction (spreading out the spectrum colors like a prism), iridescence (enhancing or canceling certain wavelengths through reflections from separate layers) and structural color. These color mechanisms are rare in the natural environment and nonexistent in artists' materials. Categories of Material Color. Using a mathematical synthesis of surface colors, I demonstrate that the reflectance profile of any material can be divided into three reflectance sections at the wavelengths 480 nm and 570 nm (the monochromatic hues cyan and yellow), within arbitrary spectrum limits (e.g., 400 nm to 700 nm). Using these spectrum boundaries, all possible reflectance profiles can be categorized into one of nine primitive color categories based on the combined reflectance within each of the three spectrum segments (diagram, below). the nine primitive color categories Note that the minimum reflectance that is greater than 0% is usually due to white scattering (W), and the maximum reflectance that is less than 100% is due to infrared absorptance (K), as explained above. The reflectance categories white, black, red, yellow, green and blue are natural language color terms in most human communities, because these colors are very common in natural or manufactured objects. The terms cyan (blue green or green blue) and magenta (red violet) are nonexistent in almost all natural language lexicons, because these colors are extremely rare in natural and manufactured objects. Nevertheless, a systematic classification of reflectance profiles requires that they be included. The long wavelength reflectance categories red and yellow are extremely common in natural materials (in particular iron oxides), define the color of all racial skin pigmentations, and are produced by incandescent materials (fire) and late day sunlight. The adjacent invisible spectral band is infrared, which corresponds to heat. The middle wavelength reflectance category green is characteristic of nearly all plants, which appear as relatively dark colors because they absorb large quantities of light; however freshly sprouted plants often appear light valued and close to yellow. The spectral sensitivity that defines our luminance perception is most sensitive in the middle wavelengths. The short wavelength reflectance category blue is characteristic of the color of the clear daylight sky and some forms of water; it is otherwise rare in natural materials, with the exception of flowers. The adjacent invisible spectral band is ultraviolet, which is almost completely filtered by the atmosphere, water and the cornea of the eye. The mixed category cyan is extremely rare at high chroma, except in certain gems (tourmaline); no language identifies this color with a basic color term. At low chroma it is a common ocean color but is identified with a compound color name ("sea green"). The mixed category magenta (kin to violet) is also extremely rare in natural or artifical materials, except in certain flowers and gems; only technologically advanced languages identify it as a basic color term. It is an extraspectral hue that do not appear in a refraction or diffraction spectrum. The achromatic colors white and black, and their mixture gray, are also extremely common in natural or artifical materials, are labeled in all language communities with basic color terms (sometimes as "light" and "dark"), and contribute to modify (whiten or blacken) the colors in every other category. Light in Space. The most important perceptual fact of light is that it occurs in three dimensional space. As a preliminary to understanding color we must, very briefly, lay out some basic facts of light in space. Illuminance & Luminance. All the material objects around us are made visible by the quantity or intensity of light that is shining on them. This quantity is called illuminance: it is the fundamental metric of lighting design for architects and interior designers, and it is measured in specific units (lux or foot candles). It is a physical fact of the visible environment around us. Surprisingly, illuminance is invisible! If we are surrounded by empty space, with our back turned to the light source, all we can see is darkness. We actually only see illuminance either by looking directly at the light source, which appears as emitted light, or by looking at surfaces around us, which display reflected light. These two subjective qualities of light are called brightness and lightness respectively  and I'll explain in a moment what those terms really mean. Now, we can only see these emitted or reflected light sources as objects in space that have a certain visual size (visual width and breadth). So the light that we actually perceive is the emitted or reflected light in relation to the visual area that produces the light. This area dependent quantity of light we can see  because it is emitted or reflected by a source of a specific visual size  is called luminance. This is the fundamental metric of photographers and video engineers, and it is also measured in specific units (candelas per square meter, abbreviated cd/m2, or the more arcane foot lamberts). Luminance Adaptation. The luminance of light reflected from surfaces to our eyes has an enormous environmental range  from a white surface under a starry sky (0.0003 cd/m2) to a white surface under noon sunlight (30,000 cd/m2 or more). This is a difference in light intensities of 100 million to 1. For purely physical reasons, no animal visual system (nor any video or photographic system) can encompass an energy range that large by a single response process. Instead, the human visual system can perceive at any time a luminance range of about 100,000 to 1, or 0.1% of the environmental range! To compensate for the missing 99.9%, the relative sensitivity of our eyes is increased or decreased by a process of luminance adaptation, which includes the familiar changes in pupil size but also requires changes in the light sensitivity of the retina and in the nerve pathways into and within the brain. These changes produce very familiar and distinct changes in the appearance of colors. Most of these changes can be handily summarized as four luminance regimes, which produce four distinct levels of luminance adaptation: scotopic or night vision, low mesopic, high mesopic and photopic or noon daylight vision (table, below). four levels of luminance adaptation adaptation level ambient light level color

experience illuminance luminance scotopic less than 1 lux less than

0.3 cd/m2 poor contrast

achromatic

no detail low mesopic 1 to 100

lux 0.3 to 30

cd/m2 low contrast

muted color

coarse detail high mesopic 100 to 1000 lux 30 to 300 cd/m2 good contrast

good color

good detail photopic more than 1000 lux more than 300 cd/m2 bright color

high contrast

fine detail As this table shows, luminance adaptation affects three fundamental dimensions of our color experience: (1) the perceived amount of contrast between white and black, (2) the perceived amount of hue purity or intensity of colors (amount of contrast between an intense color and a matching gray), and (3) the amount of detail perceptible in objects, images, textiles, textures and text.



Our visual response through all gradations of artificial light, and of natural light dimmer than full noon sunlight, is a mixture of both rods and cones, and therefore constitutes mesopic vision. Photopic vision is the experience of outdoor summer noon sunlight, and the intense artificial illumination of a surgical theater. We are normally unaware of (or ignore) the small color changes within high mesopic to photopic vision that occur in transitions across different natural or artificial illuminance environments. However, the transition from low mesopic to scotopic vision, which occurs in the hour after a cloudless sunset, produces strong and recognizable changes in color appearance (image, right). At first and briefly, reds become more luminous, as the M and S cones lose sensitivity relative to the R cone; then the R cone also becomes less senstive, and all colors decline into impoverished warm/cool contrasts (tans and browns, vs. greens and blues), and finally into shades of gray approximately one hour after sunset; parallel with these color transitions, whites decline into a luminous gray. In our homes and offices we have window blinds, and outdoors use sunglasses, to moderate light from the sun; and we use interior lighting to remedy enveloping night or a lack of windows. These adjustments clearly indicate that our preferred light regime is in the high mesopic. Your computer screen, for example, produces the "white" of this web page with a luminance of around 100 cd/m2, and the page of a book under good reading light is at least 70 cd/m2, while the luminance preferred for detail tasks in hospitals or factories is up to 300 cd/m2. I will return to luminance levels and luminance adaptation in considerations of color design. For now, the two essential points are (1) the strong interconnection between the illuminance or light level, the luminance or reflected light of surrounding surfaces, and the luminance adaptation of the eye to the surrounding luminances; and (2) the qualitatively distinct changes in color experience produced by changes in these three factors. The Illuminant. We have sufficiently anchored our perception of illuminance and luminance within three dimensional space and a specific adaptation level. Light still has two other qualities, and both have to do with color. When we think of the color of objects, we naturally think of them under a specific type of light  white light or light that is "colorless". The reason for this assumption is that the apparent color of reflecting surfaces is always the subtractive mixture of surface color and light color. In effect, light mixes with surfaces in the same way that paints mix with each other. And if we wanted to judge the hue of a yellow or red paint, we obviously would not do so by mixing it with a green or blue paint! Instead, we would mix it with white paint. Now, it seems reasonable to define a "white" light as a light source that emits every wavelength of light in equal proportion. As it happens, however, by the standard of "equal wavelengths", no natural or artificial light is perfectly colorless! Instead, lights have some color bias, which can be approximated by the mixture of a perfect white light with a colored light. The proportion of colored light in the mixture defines the chromaticity (combined hue and hue purity) of the light. color changes from low mesopic to scotopic vision A: color appearance in daylight at sunset; B-E: color changes in 15 minute increments after sunset

However, a more indirect definition than chromaticity has proven useful. For purely physical reasons, the chromaticity variations in natural and artificial "white" lights correspond closely to the apparent color of a heated metal or, more precisely, to the color of a theoretical, perfectly nonreflecting heated object called a blackbody. If we generate a blackbody spectrum in increments on the Kelvin (absolute) temperature scale from 1000°K up to 100,000°K or more, we reproduce colors only across a single chromaticity path  starting at orange red, through orange into yellow, from yellow into white and ending in pale violet blue. This curving chromaticity line is called the blackbody locus (diagram, right). (As we increase the blackbody temperature, the luminance of the material increases enormously, but we disregard luminance when thinking about chromaticity.) Then the chromaticity of all incandescent light sources can be characterized by the correlated color temperature, which is the theoretical temperature of the blackbody spectrum with the closest matching chromaticity (table, below). correlated color temperatures for common illuminants and light sources rK° color correlated illuminant or light source 1000 lower limit of blackbody curve 1850 candle flame 2000 sunlight at sunrise/sunset (clear sky) 2750 60W incandescent tungsten light bulb 2860 CIE A: 120W incandescent light bulb 3400 photoflood or reflector flood lamp 3500 direct sunlight one hour after sunrise 4100 CIE F11: triband fluorescent light 4300 morning or afternoon direct sunlight 5000 white flame carbon arc lamp 5003 CIE D50: warm daylight illuminant 5400 noon summer sunlight 6400 xenon arc lamp 6500 average summer daylight 6504 CIE D65: cool daylight illuminant 7100 light summer shade 7500 indirect northern skylight 8000 deep summer shade 9300 "white" of a computer monitor 10640 clear blue sky Sources: Mitchell Charity, MIT; Kodak USA : Mitchell Charity, MIT; Kodak USA Note: Color samples grossly exaggerate the chromatic contrast and drastically reduce relative luminance for purposes of visual illustration. The diagram (below) presents the blackbody chromaticities as locations around the hue circle. This shows that shifts in natural light correspond quite well with the traditionally defined warm/cool contrast. color analogs to daylight spectra chromaticities

the hue of blackbody temperatures illustrated as spectral locations on the CIECAM a*b* plane; solar light has a CCT of about 5500°K Keep in mind, the correlated color temperature is only useful to describe sunlight, daylight and incandescent forms of artificial light. Fluorescent lights can produce misleading results, and green or magenta Christmas tree lights would possibly have CCTs near white, which is nonsensical. Note also that these chromaticities depend on the brightness of the light: fluorescent "daylight" reading lights appear unpleasantly bluish because they are relatively dim, and a 30 watt incandescent bulb may appear distinctly yellow while a 200 watt bulb will appear white. The CCT is primarily a way to determine the chromaticity balance between yellow/red and blue, which photographers and painters use to determine the color bias of light from reflected surfaces, as this affects the adaptation of the eye, film emulsions and the apparent color of objects. This contrast is so basic that it has a simple description: CCTs lower than 6500 are termed warm, and CCTs above 6500 are termed cool. Because humans evolved in outdoor environments, we are adapted to see factually bluish daylight as "pure white" light, and the D65 illuminant is used as the daylight standard of comparison for artificial lights. Most incandescent lights produce a light that is quite yellow, where the A illuminant is a reasonable match. This highlights an important paradox in the relationship between luminance levels and color experience: as illuminance increases, surfaces appear more chromatic, but lights close to the blackbody locus appear less chromatic (closer to pure white). The Illuminant & Material Colors. The significance of the illuminant in color experience is that material colors mix subtractively with the light that illuminates them. We normally assume that the appearance of a surface color is the "real" or "true" color of the surface, because our eye has adapted to accept the prevailing illuminant as "white" light. But this obscures the consistent rule that any surface color we see is actually the subtractive mixture of the material and illuminant colors. The image (below) sets up the ideal situation: a "white" illuminant, containing all wavelengths in equal proportions, illuminates surfaces that reflect either some "green" and all "red" light (left), or some "green" and all "blue" light (right), producing the visual experience of an orange or blue color. These are in fact complementary colors, as nearly opposite in hue as color vision allows. the subtractive mixture of light and surface color the product of a single illuminant on two complementary colored surfaces; adapted from Jeff Beall, Adam Doppelt & John Hughes © 1995 Brown University However, if we now illuminate these two contrasting material colors with a "green" illuminant containing light mostly in the middle wavelengths, the effect on the visual colors is quite pronounced (image, below). restricted light emission and metamerism the product of a single illuminant on two complementary colored surfaces; adapted from Jeff Beall, Adam Doppelt & John Hughes © 1995 Brown University In the orange surface, the "red" reflectance is suppressed because there is little "red" light in the illumination, and the "blue" is suppressed by the surface absorptance. As a result, only the "green" wavelengths are reflected. The same effects apply, in complementary fashion, to the blue surface, so that again only "green" wavelengths are reflected. As a result, the orange and blue appear to be two similar shades of dull green. the blackbody locus in the CIELUV chromaticity plane

If we consider the illuminant color as acting in the same way as a tinted filter placed between the reflecting surface and the eye, then we can deduce a few consistent principles as to how this subtractive mixture of light and material color affects hues around the hue circle (images, right):  The chromaticity of the light defines a location on the hue circle, and all visual color shifts are in relation to this point.  Material colors analogous to the illuminant hue become lighter valued and more saturated.  Material colors complementary to the illuminant hue become darker and desaturated; under strongly tinted light they may appear achromatic (gray).  Material colors quadratic on either side of the illuminant hue shift toward the illuminant hue and become somewhat less saturated. The photographs below, using a camera that did not compensate for the illuminant chromaticity, show the effects of different hues of light on a material hue circle. To illustrate these effects, I've photographed a color circle consisting of the standard 18 hues in watercolor paints, illuminated by six different types of light. This procedure was not carefully controlled, but it illustrates the types of visual color changes that we encounter in materials under different types of light (image, below). illuminant and color rendering a color circle of 18 watercolor paints, plus black, viewed under daylight (equivalent to standard illuminant D65), incandescent light (A), and red (R), yellow (Y), green (G) and blue (B) spot lamps; mouseover to view colors against a neutral gray background The differences among the corresponding colors are easier to see if we extract the color samples from the "white" background and display them against a neutral background (mouseover, image above). This subtractive mixing of surface and light source also produces a fundamental color ambiguity: it is possible  and commonly happens  that (1) two different material colors (reflectance curves) can produce the same apparent color under the same illuminant, including nominally "white" light, and (2) materials that are visually identical under one kind of illumination will appear different under another kind of illumination, even if both light sources appear to provide "white" illumination. These are situations of metamerism, and the visual colors that appear different under some illuminants but the same under other illumiants are called metameric colors. Metameric colors are commonly grays and dull (unsaturated) hues; extremely impoverised or monochromatic illuminants are generally required to produce metamerisms among highly saturated material colors, and typically because all material colors then appear to be bright or dark variations on a single hue. Color Rendering. Metamerisms introduce us to a third quality of light, perhaps the more subtle. The color rendering quality of a light is simply the appearance of a complete range of colors as illuminated by the light, in comparison to the appearance of the same colors under natural daylight at the same illuminance. simulation of corresponding colors (top) nuance color circle; (middle) nuance color circle under orange illuminant; (bottom) corresponding colors under white illuminant

Daylight and direct sunlight have perfect color rendering properties both because they are broadband (all wavelengths are present) and because our visual system, eyes and brain, have evolved to accept daylight as "pure white", despite the fact that it is, objectively judged, somewhat bluish. However, sunlight under other circumstances  seen through clouds of smoke, or low on the horizon  and natural lights that are either strongly biased, or entirely missing some wavelengths from their spectrum, cause significant changes in the appearance of colors, and these push white point beyond the chromatic adaptation capability of our visual system. Daylight and all forms of incandescent (produced by physical heat) artificial light at high mesopic levels of illuminance produce perfect color rendering. Many forms of fluorescent (produced by electrical discharge) lights have poor color rendering, even when they appear to be "white" lights. This is because fluorescent lights have very uneven or "gappy" emittance profiles, so that surfaces that reflect wavelengths corresponding to these gaps will appear unnaturally dark or dull, and surfaces that reflect light that corresponds to the peaks will appear unnaturally bright. (Most supermarkets utilize special fluorescent lights to make their garden produce appear especially fresh and green.) Viewing Geometry. A final point is that visual color can depend on the spatial relationship between a surface, a light source and the eye, which is called the viewing geometry. This is defined by two completely independent quantities: the illumination geometry, measured as the angle of incidence between the light source and the surface; and the receptor geometry, measured as the angle between the surface and the line of sight to the eye, camera or photometer. These are usually notated as degrees from the perpendicular, separated by a slash: 0/45 indicates that light falls perpendicularly onto the surface, and the surface is viewed from a 45° angle at one side. Diffuse illumination, which arrives at the surface from all directions, is indicated by a lowercase d, as in: d/45. The intent of this digression is simply to make you aware that color can change signficantly due to the viewing geometry. We are all familiar with the fact that glossy or highly reflective surfaces produce a strong glare wherever the illumination and receptor geometries are equal and spatially opposite. But surfaces also generally appear whiter and more reflective (contain a larger proportion of scattered white light) if viewed from a very oblique angle (such as d/80). And both pearlescent and iridescent colors change significantly at viewing geometry changes, either by moving the light source or by changing continuously the receptor geometry (e.g., moving the head back and forth). Light Summary. Our visual experience is defined by the objective, physical quality of light in the environment. This quality comprises three basic components: (1) the light level that is created by illuminance, which produces a corresponding luminance from reflecting surfaces and luminance adaptation in the eye, (2) the illuminant or correlated color temperature, which defines the chromaticity (hue and hue purity) of the illuminance, and (3) the color rendering, which measures the difference between colors as they appear under the dominant light source and under daylight. Viewing geometry, or the relative angles of light incidence onto a surface and light reflection to the eye, may also signficantly affect color appearance, especially in glossy or highly polished surfaces, at extremely oblique angles, and in iridescent or pearlescent materials. Brightness & Lightness. We have separate terms for the quality of light that seems to be emitted by a light source (brightness) or reflected by a material surface (lightness). But if both are just forms of luminance, why do they appear to us so different? The answer is: the luminance is always perceived within a spatial context, which is just "everything that visually surrounds" the luminance in the two dimensional space of an image (on both sides, above and below) and in the three dimensional space (in front and behind) that we interpret from the image. These separate interpretations define the two basic components of color context: contrast and spatial interpretation. Luminance & Contrast. Considered only in terms of luminance contrast, brightness is luminance perceived in a context of relative brightness, and lightness is luminance perceived as a quality of relative blackness. The pivot point between these black and bright visual experiences, and the conceptual center of our luminance adaptation, is the color sensation white, the color that contains neither blackness nor brightness. Everything with luminance below white is perceived as having an increasing quality of black, and luminances above white are perceived as having an increasing quality of bright. comparison of lightness and brightness The diagram (above) summarizes this contrast as a scale and two graphs. The scale shows white as the balance point between bright and black, with the ambiguous area termed fluorescent, which can be described as light that is too faint to cast a nearby shadow. The two graphs show the key difference between lightness (a diffuse light perceived against a "white" background at 300 cd/m2), which forms a curve, and brightness (a diffuse light perceived against a completely dark background), which increases steadily as luminance increases. The lightness curve indicates that there is greater visual sensitivity to small changes in luminance in blacker (darker) values (the curve is steeper), and that lightness sensitivity extends down to a luminance that is about two orders of magnitude (102 or 1/100th) less than the luminance of white. reflectance (relative luminance) and lightness The diagram (above) expands on these points. The color that we perceive as a "middle" gray has only about 20% of the luminance (reflectance) of a pure white. If we create a 10 interval grayscale of equal 10% increases in reflectance, then the third step in the scale is a middle gray. In contrast, if we create a 10 interval scale of equal 10% increases in lightness, most of the steps appear relatively dark valued. This corresponds to the fact that, at low luminance values, small changes in luminance produce large changes in lightness, and at higher luminance values, a much larger increase in luminance is necessary to produce an equivalent perceptual change in lightness. Both brightness and lightness are contrast perceptions: they define the luminous quality of a color in relation to some benchmark or standard. Luminance & Spatial Interpretation. Because lightness is inherently a contrast perception, different types of spatial contrast can produce different visual colors. The case of two dimensional color contrasts was explored systematically in the 19th century by Michel-Eugène Chevreul, and the theory of simultaneous color contrast is well known to every artist: "In the case where the eye sees at the same time two contiguous colors, they will appear as dissimilar as possible, both in their optical composition [hue] and in the height of their tone [mixture with white or black]." These visual color changes are commonly illustrated with a "square within a square" diagram, as for example in the contrast of a dull middle blue surrounded by a similarly dull dark or light blue: color shift in a simultaneous lightness contrast all large and small squares have the same hue and chroma Here the central squares are exactly the same material color (i.e., computer monitor color), but the visual color appears darker when joined with the lighter surround (on the right), and lighter when joined with the darker surround (on the left). (Focusing your gaze on the central black dot will make this contrast more obvious.) Here is another example, using a light valued or middle valued central square and contrasting light and dark surrounds. In both cases, the surround pushes the central color in the opposite direction, enhancing the visual contrast. planar simultaneous color contrast

central squares are lightness L* = 93 (top) and L* = 59 (bottom); surrounding colors are L* = 45 (left) and L* = 83 (right) These color shifts may appear rather small. However, the images below show two identical sheets of white paper, one lying in shadow and the other in diffuse daylight. One photo shows the actual appearance; the other two photos show what happens when the color of the shadowed or illuminated paper is copied into both sheets. spatial simultaneous color contrast

a sheet of white paper viewed in shadow (left) and diffuse daylight (right)  (middle row) the original photograph; (top row) illuminated paper color copied into shadowed paper outline; (bottom row) shadowed paper color copied into illuminated paper outline The lightness differences between the central and surround colors in the two dimensional display (previous figure) excluded any effect from illumination. But the three dimensional display requires us to interpret the color area contrasts as consistent with objects illuminated by light in space. And the only way to produce a consistent spatial perception is through a radical change in the visual color. Two effects appear:  increased contrast - the comparative impact of the color differences is greater: we perceive a much greater difference between the illuminated and shadowed surfaces than in the simultaneous contrast demonstration. You may even see the "light" colored paper in shadow (image, top left) as brighter than the nearby white background of this web page.  decrement is greater than increment - copying the shadowed area into diffuse light produces a color darkening that appears greater than the lightening that appears when the lighted area is copied into shadow: the same luminance discrepancy has a much greater "blackness" (decrement) than "brightness" (increment) visual impact. And this is consistent with the response curve of lightness perception: if we take any point on the curve as the starting luminance value, a decrease in the luminance will produce a greater lightness change than will an increase in luminance by the same amount. The remarkable visual differences between two dimensional and three dimensional displays of the same color contrasts signify that color is not the quality of light reflected from materials, but the interpretation of light within a three dimensional space. We end our exploration of luminance effects in color vision by returning to the definition of color we started with. Color is a context judgment of surfaces viewed under light in space  even when the objects in space appear as a two dimensional image. Hues Within the Opponent Dimensions. Issac Newton was the originator of the hue circle, the organization of all hues as a circumference in which neighbor hues correspond to neighbor wavelengths of light in the spectrum and are judged to be perceptually similar colors. Exactly how neighbor hues are spaced in the hue circle has been an evolving standard. Newton organized his hue circle according to the apparent lengthwise spacing of hues in a prismatic spectrum (which expands the spacing of "blue" and "blue violet" wavelengths relative to the "orange red" wavelengths), and, to complete the circumference, determined the spacing between his "red" and "violet" wavelengths according to an arbitrary standard (diatonic musical intervals). Adapting Newton's scheme, 18th century artists developed the concept of partitioning the circumference into equal thirds, anchoring each division on a subtractive "primary" color (red, yellow or blue), then deriving the spacing of hues between each pair of primaries according to the relative proportions of the two primaries required to mix them. Secondary colors were for example colors mixed from equal proportions of two primaries, which placed each secondary color directly opposite the third primary on the hue circle. As all three subtractive primaries were asserted to make a "pure black" when mixed in equal proportions, the secondary colors confirmed the hue circle relationship of complementary colors also suggested by Newton: paint or ink mixtures opposite each other on the hue circle would always mix a neutral color. Eventually 19th century color scientists, using more sophisticated light measurement tools, established the perceived spacing among spectral hues as additive mixtures of three "primary" lights in a chromaticity diagram. This provided the basis for the development of colorimetry  the measurement of visual color as the measurement of material color in emitted or reflected light. Further study showed that neither the additive mixture of primary lights nor the subtractive mixture of primary paints could directly reproduce the perceptual spacing between hues around the hue circle  the apparent similarity or difference between colors judged purely by eye. This led to extended research into the perceptual spacing of colors, first as the comprehensive inventory of equally spaced color exemplars in the Munsell Color System (1929); then as complex mathematical models that attempted to reproduce the perceived color spacing entirely from the measured material color, as in the CIE uniform color space (1964) or the OSA uniform color scales (1974); and finally as fully developed color appearance models, such as CIECAM (2002), that take into account the effects of other factors (color luminance, contrast, surround colors, spatial size and luminance adaptation) on color perception. All perceptual color models developed since the middle 20th century are based on opponent dimensions that account for both the hue and chroma of all colors at equal luminance. First proposed as a scientific theory in the early 20th century (by Ewald Hering), this modern framework is based on the conceptual opponent contrast between VIOLET RED vs. BLUE GREEN (defining the a+/a dimension) and YELLOW vs. BLUE VIOLET (defining the b+/b dimension). All hues can be organized around this opponent hue circle so that the circumferential distance proportionately the perceptual difference, and visual complementary colors are opposite each other (diagram, below). the opponent dimensions and hue categories the y/b and r/g opponent dimensions and central hue locations on the CIECAM hue plane at lightness 6; dotted lines indicate area of extraspectral hues (mixtures of "orange red" and "blue violet") The use of capitals (such as GRAY) indicates that these are concept colors rather than visual or material colors, but the distribution of material colors within this space (for example, as shown here) is extremely good. The accurate reproduction of perceived color relationships is the principal benefit of these opponent dimensions. The second benefit is that color relationships are entirely divorced from any "primary" color framework. This detaches color both from the litter of 18th and 19th century "color theory" based on the concept of "primary" colors (such as Rudolf Arnheim's system critiqued above), and from any single material definition as paint mixtures or as light mixtures. The measurement of color is anchored in the fundamental habitat of color: the perceiving mind. This also does away with the specious and imaginary hierarchy among hues as primary, secondary, tertiary, pure or impure, primitive or derived, which is the third benefit of the opponent dimensions. Hues are simply hues, and (as Newton observed) all hues are equally "simple" or "homogeneal". This leads to a straightforward framework for labeling hues, based on the basic hue names red, orange, yellow, green, blue and violet and the 12 paired compound names. Three Chromaticity Spaces. The opponent dimensions allow us to map the relative appearance of many different types of material color, and this shows that visual color can be separated into three distinct color spaces, defined not by hue or by brightness/lightness, but by the maximum hue purity (chroma or saturation) any color can achieve. These three domains are (1) the physiological limits of the retinal photoreceptors, produced by monochromatic lights or spectral hues, (2) the ideal limits of perfectly reflecting colored materials, defined as theoretical optimal colors, and (3) the actual limits of the most saturated pure pigments or dyes displayed in a transparent medium, or media gamut. The diagram (below) shows the relationship among these three color domains on the CIECAM a C b C (chroma) plane. three chromaticity spaces visualized on the CIECAM a C b C chromaticity plane The physiological limit in color appearance is traced by monochromatic lights or single wavelengths from the visible spectrum. Displayed at optimal contrast against a dark neutral background, no physical stimulus can produce a more saturated color appearance or a wider range in brightness. The definition of these physiological limits depends only on the trichromatic L, M and S outputs. The limits are partly constrained by the fact that a light stimulus projected onto the retina will always stimulate more than one type of cone, and this receptor "mixture" reduces the color saturation. The ideal material limit of surface colors is identified through the mathematical fiction of optimal colors, which are ideal materials that either completely reflect or completely absorb all light at each wavelength. This produces ideal surface colors, in a complete lightness range from absolute black to luminous white, that are as saturated as possible in every hue across the full range of lightness. Their limits are established by the fact that the lightness of a material increases from pure black as the proportion of reflected wavelengths is increased, but these multiple reflected wavelengths are mixtures of single wavelengths that produce colors less saturated than single spectral lights. However, unlike actual material colors, these theoretical colors entirely exclude any whiteness from surface scattering, and have very sharply defined reflectance profiles.

The actual material limit is determined by the interactions of matter with light. Because they never completely absorb or reflect all the light at each wavelength  physical surfaces channel some luminance into invisible infrared wavelengths and scatter some as diffuse "white" light, producing rounded and darker reflectance profiles  they are inherently duller than optimal colors of the same hue and lightness, and they have a more restricted lightness range. Material color has a characteristic quality of grayness, the combination of white scattering and black absorptance, even in a white color. This grayness is the difference between material and optimal color. These three chromatic domains highlight the important and obscure relationship between brightness/lightness and chroma. Both depend on relative luminance. As explained above, the perception of both hue purity and brightness/lightness depends on whether, and how, we take into account the effects of illumination and luminance contrast. This distinguishes them both from perceptions of hue, and positions both brightness/lightness and chroma as symbols or signs of higher luminance. In fact, it is possible using luminance contrast alone to make a surface color appear to glow or shine like a light, with a corresponding increase in its apparent chroma (images, right). Art galleries exploit this fact to make their paintings appear more luminous, vignetting them with faint spot lights that enhance luminance contrast with the subdued illumination of the gallery space. The Nuance Space. Another indication of the relationship between luminance and chroma is that each hue can reach its maximum chroma only at a specific lightness. For example, yellow only reaches its peak, sunny chroma at a very high lightness; at darker values it becomes a dull umber or green. Blue, in contrast, reaches its peak chroma only at low values; at lighter values it appears pale or whitened. As we have seen, many systems of color harmony  the Chevreul system, the Munsell system, the Ostwald system, the Coloroid system  have recommended harmonies based on nuance, different hues of the same lightness and chroma. But what happens if we want to find nuance harmonies between yellow and blue? The illustration below superimposes two pages from the Munsell Book of Color for a warm hue (middle yellow, 5Y) and a cool hue (middle blue, 5PB). overlap in two hues from the Munsell Book of Color a - the peak chroma of blue (at value 4); b - the peak chroma of yellow (at value 9); c - the highest chroma blue at a value equal to b; d - the highest chroma yellow at value equal to a; e - the maximum chroma at which yellow and blue have the same value and chroma; f,g - colorant nuance of blue and yellow (see diagram below) As the diagram suggests, there is a large area of colors (below the orange line) where both a yellow and a blue can be found at matching lightness and chroma, which produces a nuance match. But these nuance matches exclude the peak chroma of both colors, and nearly all the chroma at the light and dark extremes. We end up with roughly a pyramid of chroma, where the most saturated nuance match is near a middle lightness of 6 on the Munsell value scale (point e). luminance contrast and

color chroma an identical burnt sienna can create visual colors orange, brown or black solely by manipulating the luminance contrast with its surround

In fact, if we consider all hue overlaps in this way, the green intersection between yellow 5Y and blue 5RB (above) defines the area in which a nuance match between any two or more hues can be found  that is, the volume in which every hue can be represented at the same lightness and chroma. I call this the nuance space, which can be defined in terms of optimal colors so that it is completely media independent (diagram, below). the nuance space the range of lightness and chroma values common to all optimal color stimuli, expressed in units of Munsell chroma (left) and average CIELAB chroma (right), on a vertical CIE L* scale (grayscale) Measured within the chromaticity space created by optimal colors, the most saturated possible surface colors, the nuance space has roughly a diamond shaped cross section. It is widest just above a middle gray (L* = 50 to 65), and tapers above and below into the achromatic axis at pure white and pure black. The nuance hue circle at peak chroma is approximately at Munsell value 6 or CIE L* 60, a middle gray. To illustrate the range of chroma this hue circle encloses, here are the Munsell aim colors at value 6 (Lab lightness 62) and chroma 14 (Lab chroma between 60 and 100). a nuance hue circle the Munsell hue circle at V = 6 and C = 14, displayed on a gray V = 4; central gray circle is also V = 6 The nuance space shown above defines the perceptual limits of nuance combinations; the color space defined by actual pigments or dyes, or by the gamut of a display or printer, will be smaller and less regular. In other words, there are two different ways to implement the concept of nuance  as perceptual (media independent) chroma limits, and as colorant (media specific) chroma limits. These two approaches yield different nuance color harmonies, as shown in the diagram (below). two types of nuance perceptual nuance based on CIELAB L = 50 and C = 50 across all hues; geometrical nuance based on (a) saturation = 85% and brightness = 50% across all hues; (b) saturation =60% and brightness = 75% across all hues; (c) saturation=75% and brightness = 80% across all hues; dots indicate identical colors in perceptual and geometrical nuance rings; letters f and g identify yellow and blue colors in the Munsell diagram (above)  perceptual nuance equates hues that have the same perceived lightness and chroma, or brightness and saturation, as defined in a perceptually based color model such as the Munsell Color System, the OSA uniform color scales, or a CIE color model (such as CIELAB).  colorant nuance equates physical color samples of pure color pigments subtractively mixed with the same relative proportions of white and black pigments, as exemplified in the "triangular" geometries of the Swedish NCS or DIN color models; or it equates printer or display colors with the same absolute proportions of additive brightness and saturation, for example as in the Adobe Photoshop color picker. Colors included in a single perceptual nuance vary significantly in media specific brightness and saturation, which means different hues in a perceptual nuance ring anchor or lead into different colorant nuance rings (diagram, above). In the same way, a colorant nuance ring must contain hues of different perceptual nuance. Obviously, colors in a colorant nuance ring will differ visually from one another more than they do in a perceptual nuance ring, because visual color rather than material color is the ultimate framework for color appearance. Achromatic Luminance vs. Chromatic Luminance. We now can return briefly to the luminance dependence of lightness and chroma. Within the nuance space, chroma and lightness are mutually restrictive: if we want high chroma contrasts across all hues, then we are limited to the lightness range around middle grays; if we want high lightness contrasts across all hues, then we are limited to the chroma range near grays. Outside the nuance space, chroma still restricts the range of lightnesses, in the sense that high chroma confines the lightness range to very light values (for yellow and orange hues) or very dark values (for blue and violet hues). Hues by comparison comprise a collection of vibrant individuals, which may explain the emphasis on hue categories, sometimes to the exclusion of lightness or chroma, in traditional formulations of color harmony and color design ("yellow is the complement of purple", "blue and yellow make green", etc.). In any case, like a schoolroom of children, hues require discipline and direction, and to that purpose lightness and chroma exert a powerful controlling effect. Of the three colormaking attributes, lightness is the dominant element in visual design, and contrasts in lightness are the single most reliable and potent method to produce visual contrast. In addition, achromatic lightness variations almost always appear "harmonious" or acceptable in any combination and for any purpose, provided they serve their function  for example, the contrasts do not obscure a pattern or text. The "cost" here is that hue is suppressed. At the other extreme, chroma amplifies all hues to their peak individuality, but at the cost of making lightness variations more difficult to control, in the dual sense that the peak chroma dictates a specific lightness for any hue, and that chromatic luminance adds to the apparent achromatic luminance (saturated colors appear "brighter"), which obscures the underlying lightness relationships or "value design". relationship of lightness, hue and chroma in color design

A key image therefore is that chromatic hues are poised opposite achromatic values in a balance, and the fulcrum is defined by the relative contribution of chroma to the color variations. As chroma increases lightness loses leverage in the color design in relation to hue; as chroma decreases, hue loses leverage in relation to lightness. Context & Pattern. Separate from the sensual qualities of color are its properties as pattern. Visual Area. The angular dimensions of something as measured from the viewer's eye, the height and width of something relative to the entire field of view, the visual size. Spatial Frequency. The "spacing" or visual size of color areas. In three dimensional space, spatial frequency (resolution) decreases as the color area is farther away or physically smaller, and increases as the color area becomes closer or physically larger. Spatial frequency is most familiar through the process of visual fusion, which causes colors to mix (in additive color mixture) as the color areas get smaller. Increasing distance in space transforms the appearance of objects into structurally or visually related textures. And at extreme distances, texture itself dissolves into pure color (diagram, below). visual fusion of three different color pairs The are at the top of the image is made of the same green and red pixels as the area at the bottom, but the color units are too small to see individually: instead they mix visually to make yellow or gray. There is a fusion threshhold for every texture, beyond which it is blended by the eye into a single homogenous color. Color TV screens, a distant mountain slope and a sandy beach are all composed of tiny discrete forms beyond the visual mixing threshhold. However, visual fusion is not the only spatial effect in color perception. Color roughly divides into three domains: 10° (wide field) and 2° (foveal) chromaticity diagrams  10° or wide field color, within a visual area roughly the size of an orange (or larger) held at arm's length, a common industrial standard for color perception, determines our perception of any large color area  a wall, a carpet, an automobile, a vase, a book cover 3 and is only weakly affected by contrast with surrounding color areas.  2° or foveal color, within a visual area roughly the size of a 1 Euro coin or a US quarter held at arm's length, is a common colorimetric or color measurement standard, is somewhat limited in the perception of blue colors and significantly affected by contrast with surrounding color areas.  pixel color, within a very small visual area (such as one pixel in your computer monitor) produces highly degraded color that can be almost completely insensitive to blue hues and very strongly affected by contrast with surrounding color areas. Visual fusion operates in three dimensional space to produce the following sequence of contrasts between large vs. small or near vs. far visual elements: pattern > texture > color Pattern is the covering of a visual area by means of the tiling or repetition of homologous, visually recognizable and smaller color areas. Pattern is always an artificial surface, expressly created for its visual impact. Texture is the covering of a visual area by means of the random or irregular distribution of very small color areas, or by a uniform repetition of surface irregularities. Texture is typically a natural surface or uncontrolled variation in a manufactured surface, and typically the visual appearance is a report of its material composition or its tactile quality. Text. Text is the arrangement of standard units of pattern to symbolize language. As pattern, text has been frequently used in architecture, graphical arts and painting, from the Renaissance to the present day, as both decorative and thematic content. A Color Design Vocabulary. To conclude this long tutorial, I must address the logical gaps created by reliance on only the traditional terms harmony and contrast in previous color theory. In particular:  color principles have been formulated in terms of concept colors, and are therefore detached from the material variation of a particular object or environment  color is defined as colorant relationships (mixtures of three primary colors) instead of as visual (perceptual) relationships; and  design principles have been formulated without regard to the purpose or place in which the colors are displayed. Let's step back from "color" harmonies defined along the color circle  which are really only hue harmonies  and look at these simple contrasts as "full color" problems including lightness and chroma. In an earlier section, I proposed that: Color harmony is the manipulation of lightness and chroma within a given selection of hues so that all colors contribute to an intended visual effect. This is a break with traditional color theory, where the emphasis is on carefully selecting hues  "the color in colors"  to produce a color harmony. Apart from any innovations or insights into color perception or color physics, it seems very clear that any advances in "color theory" will require a more precise and flexible vocabulary to denote the kinds of effects we want to describe as color principles. Herewith my attempt to fill that need.  design frame is the visual scope of color evaluation; the boundary around visual design. For a vase as an object, the scope is the vase itself; when the vase is used as an element of interior decor, the frame is the environment in which the vase is displayed.  color palette is the enumeration of dominant or frequent colors presented within a frame; the countably different colors (defined in terms of lightness, hue and chroma, rather than just hue) that appear within the frame. For a painting, the palette is the enumeration of the raw pigment colors used to mix all the other colors in the painting.  color dimensionality refers to the number of colormaking attributes varied within a palette. A palette that varies only in lightness (monochromatic or achromatic) or hue (nuance) is unidimensional, a palette varied only in hue and lightness, or hue and chroma, or lightness and chroma, is bidimensional; a palette that varies on all three colormaking attributes is multidimensional. Multidimensional palettes include palettes multiplied through pigment mixture, or through complex patterns or textures.  color contrast is the visual dissimilarity between all colors in a palette. This is approximately equivalent to the distance between colors in a perceptual, uniform color space, although the ultimate criterion is the visual color appearance, and not the representation of the colors in any abstract geometry. Colors that provide the largest contrast are widely separated in lightness and chromaticity: strongly saturated colors very different in hue and lightness.  color volume is the contrast multiplied by the dimensionality of a palette, the number of dimensions in the palette and the size of the color contrasts produced by the contrasts on those dimensions.  color center is the color identified as the average lightness and chromaticity of all colors in a palette, when the colors are combined according to the Newtonian method of weighted averaging.  color unity is a small volume of pairwise contrasts within a color palette, typically produced by a common property of light, material or pigment. Unity subdues the visual variety in an architectural space, or the visual impact of a pattern, design or text.  color variety is a large volume of pairwise contrasts within a color palette.  color consonance is present when each color appears as or more saturated and luminous when viewed within the color field than when viewed separately on a gray background; color dissonance is present when one or more colors seem to produce an unwanted darkening or dulling in the appearance of other colors in a palette.  color gradation is the number of discrete color mixtures produced by a color palette; gradation is common in paintings and in many kinds of natural materials.  color balance means that no hue appears more dominant or more prominent than any other, by reason of its lightness, chroma or visual area.  color coherence means that all colors are fitted to an underlying pattern, design or text in a way that makes the underlying pattern visually distinct and legible. I have intentionally excluded the term harmony simply because it has been used over the past three centuries with so many different denotations, and with relationship to so many different theories of color design, that it has become hopelessly ambiguous, if not misleading. Visual Color Examples. Finally, it is important actually to look at different combinations of lightness, hue and chroma in order to identify valid principles of color design. For that reason I have created several large image files. These 90 image files are online as a library of chromatic induction & contrast. Readers may also want to browse my image recreation of the OSA Uniform Color Scales, as these images present arrays of evenly spaced colors as differently oriented slices through the perceptual color space. principles of natural color harmony With the key concepts in hand, let's now formulate the essential principles of a natural color harmony. Before you tackle this section, you should be familiar with the three colormaking attributes. Correct understanding of these terms is essential to the color differences I will discuss. In particular, hue names refer to colors as they appear in the visible spectrum, including extraspectral purples and red violets. Brown, ochre, pink, olive, etc. are not hues but colors produced by darkening or whitening red or yellow hues. Also, remember that a color contrast viewed on a light emitting computer monitor may not have the same effect as a contrast viewed in light reflecting paints or colored papers. If possible, print out the color examples and compare the visual impact of the paper and monitor versions. An Empirical Framework. In response I've developed a theory of natural color harmony that organizes color in relation to five components (figure, below). the five components of a natural color harmony These components are:  illuminance - What is the overall intensity of light in the environment where colors are viewed?  illuminant - What is the chromaticity (hue and saturation) of the light? What are the color rendering properties of the light? How strongly does the chromaticity contrast with the light in immediately previous light environments?  material variation - What are the materials defining the built environment, the surface environment, the objects in the environment, the landscape or setting? What is their natural, customary or habitually experienced range of colors, defined as a gamut and a palette of surface textures and physical weights?  context - What is the color for? How will the color function or be interpreted in relation to the criterion context  as surface pattern, as unique object, as representation or as spatial environment (PORE)?  perceptual structure & response - How will the average person perceive the colors in the given situation, in terms luminance adaptation, contrast, color associations and cultural conventions? My alternative natural color harmony unfolds from the answers to one question: how do colors actually combine in everyday perception of the real world? I believe these familiar color combinations help to establish color harmony attributes by shaping the involuntary responses of the eye to light and of the mind to color. I've come to believe that our sense of color harmony is fundamentally based on the effects of light on color appearance, on the eye's response to light and color through chromatic adaptation, and on the color contrasts that are most important in our perception of surface patterns and spatial relationships. Foundations of Natural Color. 1. Color is a contextual interpretation of sensory information. This is basically a caution against thinking about color design in terms of concept colors, those abstract, language based, memory anchored color ideas that are neither physical materials nor visual sensations. We can certainly summarize a desired color in words, or request an ink mixture using a CYMK formula, but we must always anchor our color designs in the actual visual inspection of the color combinations produced by specific materials examined under the intensity and color of light that will be used for their long term display. 2. Color interpretations build on our lifetime experience with all physical environments under all types of light. In part this cumulative experience is "hard wired" into our visual system, as our infant visual system developed into childhood; in part it is "soft wired" as memory and conception; and in part it is "dynamically wired" as the visual and cognitive capability to compensate for and ignore variations in natural light and variations in the color of surfaces due to shadows, artificial lights, foggy days and so on. 3. The biological adaptations for color perception have been determined by evolution to natural surfaces under natural light. A substantial amount of the foundation structure of our vision has been inherited from our primate ancestors and, through them, from our mammalian and even vertebrate ancestors. Human photopigments (in fact all mammalian photopigments) evolved from the photopigments of prehistoric fishes. All these animal visual systems evolved in relation to terrestrial materials under solar light, and these form the bedrock and skylight outlines of color experience. 4. Some part of the coappearance of colors is due to the physical structure of the natural environment and the qualities of natural light. 5. Some part of the coappearance of colors is due to perceptual structure evolved in relation to natural surfaces under natural light. Complementary shadow colors, contrast and luminance, 6. Modern color experience is a mixture of both natural and artificial surfaces under both natural and artificial light. 7. Some part of the coappearance of colors is derived from the constructed environment and the effects of artificial light. 8. Colors that consistently coappear in color experience are perceived to "go together" in artificial color arrangements and color design. We accept the natural world as natural, and find nothing objectionable in its color combinations  though we may recoil from the substances the colors signify! 9. Colors that have been habitually perceived to coexist in specific environments are learned as significant signs of those environments. Environments have a deep structure that affects their color palette. Artic environments are characterised by copious frozen water; deserts by an abundance of silicates and iron oxides; forests by plants and trees; tidal pools by ocean water and foam. In each environment the light of the sky is different, due to differing combinations of humidity, suspended ice crystals or dust, and the filtering of light through leaves or reflected from wet surfaces. 10. Three forms of color response  the biological, the natural, and the artificial  coexist and dynamically influence one another in color experience. The previous principles make it clear that we cannot account for color harmony or contrast solely in terms of "primary" color combinations, or color series within a perceptual color solid, or the coappearance of colors in nature, or any other one dimensional color harmony scheme. Color is a densely determined experience, and we gain control of color to the extent that we are able to take its various dimensions into consideration. The Perceptual Natural Color Hierarchy. Colors do not appear in our experience with equal frequency across all light environments and in completely random asociation with other colors and surface materials. Instead, our color experience is structured by the natural and artificial environment, and our cumulative color experience forms the foundation of our sense of color harmony. X. Novelty is the cumulative frequency of a specific visual color experience within the totality of color experience. Every material color, across all variations in illuminance and illuminant, defines a group of visual color experiences. These experiences form part of the total color experience across the lifetime of the individual. Material colors are determined by the facts of physical reality  the structure of molecules and crystals, the density of surfaces and films  X. Color novelty is largely perceived in relation to the color experience provided by recognizably distinct physical objects