Stare at a piece of colored paper for 45 seconds or so. Then look at a plain white surface. The cones in your retina that are receptive to the color of the paper become fatigued (depleted of a light-sensing chemical that replenishes slowly), so you see an afterimage of the complementary color when you look at a white area. This is because the fatigued cones send a weaker signal to the brain. All the colors produce white, but all the colors minus one produce the complement to the missing color. Try it and see!