Impossible color

An impossible color is a color experience that ordinary human vision cannot normally produce, such as a single hue perceived as both reddish and greenish.

Opponent-process theory: A theory of color vision in which visual signals encode opposing pairs such as red–green and blue–yellow. Its opponent channels normally prevent red–green and blue–yellow from being experienced as single hues.
Reddish green: A proposed hue experience combining redness and greenness at the same location and time. It is the best-known example of an impossible color in opponent-color discussions.
Ewald Hering: A German physiologist who proposed that color vision is organized around opposing red–green and blue–yellow processes. His opponent-color account supplied the central framework behind the notion of incompatible hues.
Color naming: The use of words to categorize and communicate color experiences. Verbal reports alone cannot establish whether observers experience a genuinely novel hue.
Color opponency: The neural encoding of color through channels that compare activity associated with opposing hues. Opponency is the specific neural arrangement that makes these hue combinations difficult to perceive.
Yellowish blue: A proposed hue experience combining yellowness and blueness at the same location and time. It is the other classic opponent-hue combination claimed to resist normal vision.
Johann Wolfgang von Goethe: A German writer and natural philosopher whose color theory emphasized perception and the role of the observer. His work helped establish a tradition of treating color as a perceptual phenomenon, not merely a physical spectrum.
Visual adaptation: A change in visual sensitivity or appearance caused by sustained exposure to a stimulus. Adaptation can create unusual aftereffects that may be mistaken for impossible colors.
Retinal ganglion cell: A retinal neuron that sends visual information from the eye toward the brain. Some retinal ganglion cells carry opponent signals that contribute to the constraints on hue perception.
Chimerical color: A color aftereffect produced by adapting to one stimulus and then viewing another. Some reports describe chimerical colors with combinations resembling impossible hues.

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