McCollough effect

The McCollough effect is a phenomenon of human vision: Colorless gratings appear colored contingent on the orientation of the gratings. People need time to see the effect. For example, if someone looks at a red horizontal grating and then at a green vertical grating for a few minutes, a black-and-white horizontal grating will look greenish and a black-and-white vertical grating will then look pinkish. The effect is remarkable for often lasting up to three months or more.
The effect was discovered by American psychologist Celeste McCollough in 1965.