Opponent Processing Theory of colour vision
The opponent-colours theory of colour vision, proposed by Hering, seemingly contradicts the Young-Helmholtz trichromatic theory. It was advanced to explain various phenomena that could not be adequately accounted for by trichromacy. Examples of such phenomena are the after-image effect (if the eye is adapted to a yellow stimulus the removal of the stimulus leaves a blue sensation or after-effect) and the non-intuitive fact that an additive mixture of red and green light gives yellow and not a reddish-green. Hering proposed that yellow-blue and red-green represent opponent signals; this also went some way towards explaining why there were four psychophysical colour primaries red, green, yellow, and blue and not just three. Hering also proposed a white-black opponency but this third opponent channel has been abandoned in most modern versions of the theory. It is now accepted that both the trichromatic theory and the opponent colours theory describe essential features of our colour vision with the latter theory describing the perceptual qualities of colour vision that derive from the neural processing of the receptor signals in two opponent channels and a single achromatic channel.