Trichromacy is a special feature of the visual system whereby three independent types of photoreceptors within the retina are “tuned” to differentially respond to different wavelengths in the visible spectrum of light1. Trichromatic color vision characterizes humans among other animals and is postulated to have evolved to improve foraging; in particular, the specific set of pigments expressed in the human eye enhances differences between red and green nuances2,3,4. The evolutionary advantage would reside in the fact that more reddish nuances in fruits and leaves generally indicate higher energy or greater protein content5. In line with this idea, trichromatic primates relay as a default on sight more frequently than scent when making decisions about food and show a preference for food with more reddish nuances. Experimental evidence also supports this idea, showing that trichromatic primates are better off than dichromates in judging ripeness of fruits and edibility of leaves6,7,8,9. However, it is not fully understood how and to which extent color plays a critical role in humans’ food choice and, in particular, whether trichromatism guides visual evaluation of nutritional and appetitive properties of food (see refs 10, 11, 12, 13, 14 for evidence of differential brain activations as a function of perceived calorie content of food images).

With the notable exception of color effects on taste/flavor identification and intensity judgments15, the possible role of color in food evaluation has received relatively little attention in past research maybe because the human diet is not limited to fruits and leaves found in nature. In fact, many food items eaten by humans normally undergo some form of transformation like cooking (see ref. 16). This is a human ability that is common to many different cultures that cook their food in a variety of ways, ultimately changing the visual appearance and color of food as well as making its energy content more accessible17. Indeed, cooking has been argued to represent an evolutionary advantage because it may have provided the necessary surplus of energy needed to support larger brains (see ref. 18). The observation that great apes tend to prefer transformed food, although they never developed cooking19, is taken as evidence that hominids too preferred transformed food18. Likewise, a study with mice17 revealed that fasted animals naturally preferred cooked-food diet and that the animals lost less body weight on a cooked diet than non-cooked diet, suggesting a greater energy intake with the former diet, with food quantity being equal.

In the present study we aimed at testing whether red/green color shades are associated with food evaluation and preference in modern humans. The trichromatic vision, by improving foraging in primates, may still bias humans to rely heavily on sight for food evaluation. More specifically, we hypothesized that red and green brightness would selectively and inversely predict arousal and calorie estimation of any food. Irrespectively on the type of food, we expected individuals to prefer red- over green-looking natural food on the ground that this strategy has been found successful for non-human primates, to such an extent that human eye pigments are tuned to best perform the red/green discrimination. In the case of transformed food, where cooking changes its color as well as its calorie content, color nuances might not be an efficient cue to extract information about the nutritional content. Nevertheless, irrespective of the level of transformation it underwent, we hypothesized that humans would prefer more red food items over more green food items. In other words, we hypothesized a bias towards more red-nuanced food items and bias against more green-nuanced food items.

To test our hypotheses, we asked healthy participants to rate how arousing they perceived a large set of food and non-food images (see Fig. 1). Previous research indicated that arousal is a proxy for motivational value toward an object in particular food20. Arousal, in fact, predicts wanting21 and mediates preparatory behavior20. Thus, we used reported arousal as a proxy for the motivational value of an object. We independently estimated calorie content of food stimuli and asked participants to rate the perceived calorie content. Since we argue that the effect of color on arousal and calorie content derived from the relationship between energy-content and color in natural food, we also asked participants to rate the level of transformation of different food images, and the work required to prepare them so that this variable could be accounted for. Participants’ characteristics (e.g., Body mass index, hunger level) and visual properties of each image (e.g., size, spatial frequency) were also assessed. Multiple regression analyses served to assess the relevance of each predictor.