Mankind as a species has little reason to boast about his sensory capacities. A dog’s sense of smell, a bat’s hearing, and a hawk’s visual acuity are all superior to own. But in one respect we may justifiably be vain: ability to see colours is a match for any other animal. The world seen in monochrome would be altogether a drearier, less attractive place to live in. But Nature did not grant colour vision to human beings and other animals simply to indulge their aesthetic sensibilities. The ability to see colour can only have evolved because it contributes to biological survival. Experiments on colour preference in humans have given results which appear at first sight to be at odds with those in other primates. The explanation of red’s psychological impact must surely be that red is by far the most common colour signal in nature. There are two good reasons why red should be chosen to send signals.