Seeing red…and yellow…and green…and: we owe our appreciation of color—what it is and how we perceive it—to scientists and artists. Do we also have some hungry primate ancestors to thank for the great pleasure it brings us?

The difference between two- and three-color vision is significant. In the dappled light of a rainforest, primates with dichromatic vision may fail to distinguish between mature green leaves (which are rarely eaten) and ripe, pulpy orange and yellow fruits such as bananas. By contrast, the peak sensitivities of the three cones in the trichromatic system are well placed to provide good visual discrimination between these colors and therefore to help monkeys find their food. If our color sensitivity were spread more evenly across the spectrum, we would have smoother color vision but would have a harder time distinguishing colors in the orange-yellow-green range--distinctions crucial to our hungry tree-dwelling ancestors. So it can be argued that we visit art galleries with a visual apparatus that's fine-tuned to locating bananas. Perhaps Andy Warhol was on to something.

Some researchers think that linking primate color vision to fruit eating doesn't explain everything, however. Besides eating fruit, primates consume leaves, nuts, insects, and other prey. Since young, succulent leaves in rainforests are often reddish, folivory may have provided another impetus for color vision to evolve as it did, providing us with a firm red-green distinction. The New World howler monkey, which has a notably folivorous diet, evolved full trichromaticity independently of Old World primates.

Dichromatic humans--who lack either L or M cones--are unable to experience the lush diversity of hues in fall leaves. But dichromaticity has advantages, too. It appears to confer an improved ability to distinguish textures and to spot camouflage. Dichromatic monkeys are therefore good at detecting insects--or predators, for that matter--that try to blend with their surroundings. And some important fruits, such as figs, are colored like the surrounding foliage but are distinguishable by their texture. Social animals such as monkeys may thus benefit from foraging in groups that contain both dichromats (the males) and trichromats (some females). Humans have discovered the same thing in adverse circumstances: during the Second World War, the British Royal Air Force used color-blind dichromats to spot camouflaged camps and vehicles in reconnaissance flights over enemy lines. Paradoxically, nowadays colorblindness can rule out a career in the RAE A good eye for color may be invaluable in the gallery, but in the wild it may be a mixed blessing.

BLUE

Julius Caesar's legions were awed by the fierce blue warriors who resisted the Roman conquest of Britain. "All Britons dye themselves with woad which makes them blue," Caesar recorded, "so that in battle their appearance is more terrible."

Woad was extracted from the plant Isatis tinctoria, which grew throughout Europe and Asia. Its blue colorant is chemically identical to the indigo dye made from plants of the genus Indigofera, cultivated in Asia. The Romans themselves imported the dye from the East and used it to paint their armies' parade shields.