The McCollough effect is a phenomenon of human visual perception discovered by American psychologist Celeste McCollough in 1965 in which colorless gratings appear colored 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 is remarkable because it is very long-lasting. McCollough originally reported that aftereffects may last for an hour or more, but they can persist much longer. A 1975 study found that 15 minutes of induction can lead to an effect lasting three and a half months.
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The Daily Omnivore 