Life is full of aphorisms ascribing properties to particular colours. The scarlet woman. Red light spells danger. Yet, according to the first in the new series of Horizon, colour is “one of nature’s great illusions”. Even so, wearing red reduces stress and increases confidence. This examination of how colour is seen and interpreted, and how it affects us, revealed that an awful lot of science bods are bothered about how and why we see what we see.