High Quality Content by WIKIPEDIA articles! The McCollough effect is a phenomenon of human visual perception in which colorless gratings appear colored depending on (contingent on) the orientation of the gratings. It is an aftereffect requiring a period of induction to produce it. For example, if someone alternately looks at a red horizontal grating and a green vertical grating for a few minutes, a black-and-white horizontal grating will then look greenish and a black-and-white vertical grating will then look pinkish. The effect was discovered by Celeste McCollough in 1965.