The color phi phenomenon is the fact that, when apparent motion is induced between objects with different colors, the color of the apparently moving object abruptly changes midway along the path. It is a perceptual illusion described by psychologists Paul Kolers and Michael von Grünau in which a disembodied perception of motion is produced by a succession of still images. The color phi phenomenon is a more complex variation of the phi phenomenon. Kolers and von Grünau originally investigated the phenomenon in response to a question posed by the philosopher Nelson Goodman, who asked what the effect of the color change would have on the phi phenomenon.