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.