Read that backward and you get Roy G. Biv (the I is missing above, but close enough). That’s the mnemonic device for remembering the colors in the visible spectrum of light, or in another sense, the colors of rainbows. Red, Orange, Yellow, Green, Blue, Indigo, and Violet. And in between each of these colors is almost every other color we can detect.

Almost every other color — with “almost” being the operative word.

The image above is magenta. It’s nowhere to be seen in the spectrum at the top, though. Magenta (often called pink), just isn’t there. But, color-blindness aside, we can clearly see it. What’s going on here?

First, let’s talk about the rainbows — the spectrum depicted at the top. Light comes in all sorts of wavelengths, and us humans can detect light in many of those wavelengths. (We can’t see all of them — infra-red and ultra-violet are probably two of the more commonly known invisible ones, but radio waves, x-rays, and gamma rays are also examples.) The light itself doesn’t actually have a color — as Isaac Newton observed, “the rays, to speak properly, are not colored. In them there is nothing else than a certain power and disposition to stir up a sensation of this or that color.” Our brains just associate different wavelengths with different colors. The range of 380 nanometers to about 450 nanometers are seen as various shades of violet, for example. Magenta, though, doesn’t have an associated wavelength.

Instead, our brain just kind of makes it up when other information comes in.

Our eyes have photoreceptor cells called rods and cones. Rods detect light generally, even in small amounts, but cannot help us determine the color of things. Cones, which require more light before they turn on, help us figure out the colors. (That’s why in dark areas, we often can’t tell what color things are.) Humans have three types of cones — red, blue, and green. Everything the cones detect, therefore, is actually just one of those three colors, and our brains fill in the gaps so we can “see” the other colors of the rainbow. When a yellow wavelength comes in, for example, the red and green cones are triggered. Our brains interpret that as “yellow” and bananas, school buses, and lemons are better off for it. This makes sense, looking at the rainbow above — if you look between red and green, you’ll see yellow is situated right in there.

Magenta occurs when the red and blue cones are stimulated. That’s a problem if you look at the rainbow, because there’s no “between” red and blue, as the ends of the spectrum don’t connect with each other. (As this video points out, the spectrum is a line, not a circle.) The brain needs to do something with that information, and magenta seems like a pretty good solution, although for no obvious reason. After all, as Scientific American said (echoing Newton’s observation), color “is all in your head [. . .]. It is a sensation that arises in your brain.” If we’re going to make up the colors anyway, there’s no reason to limit ourselves to the stuff found in the visible spectrum.

Bonus Fact : As noted above, when our eyes detect yellow wavelengths, that light is captured by the red and green cones and translated into what we think of as yellow. Most computer monitors (and TV and smartphone screens, too) take advantage this conversion process and skip the first step — there’s no yellow wavelengths being used whatsoever. (That’s also true for cyan, brown, and of course, magenta.) All the colors the monitors show as are actually just a mix of red, blue, and green light. If you could magnify your screen, you’d see something like this, and you’ll notice that there are only three colors there. Interestingly, that means that our computers are in a sense tricking our brains into seeing things which aren’t really there (with the above-mentioned caveat from Newton still being true). Video blogger Vsauce explains more on the difference, here.

From the Archives: Invisible Pink: How the Royal Air Force (UK) used pink as camouflage.

Related: Blue’s friend Magenta.