Complementary color
If a person stares at a single color for about a minute then looks at a white surface, an afterimage of the complementary color will appear. This is because of eye fatigue. For example, if the person stares at a red color, the photoreceptors (cells in the eye which catch colored light) for red light in the retina (the back part of the eye) become fatigued.  When photoreceptors are fatigued they are less able to send information to the brain. If the person then looks at white light, all photoreceptors will send information.  Because the photoreceptors for red light are fatigued, the information they send will not be as strong as the information about the colors other than red and the illusion of seeing the complementary color is created.