- Reddish Green redirects here. Or see Reddish (an area of the Metropolitan Borough of Stockport, in Greater Manchester, England).
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Impossible colors or forbidden colors are hues that cannot be perceived in ordinary viewing conditions from light that is a combination of various intensities of the various frequencies of visible light. Examples of impossible colors are bluish-yellow and reddish-green. This does not mean the muddy brown color created when mixing red and green paints, or the green color from mixing yellow and blue paints, but rather colors that appear to be similar to, for example, both red and green, or both yellow and blue. Other colors never experienced in ordinary viewing, but perceivable under special artificial laboratory conditions, would also be termed impossible colors.
Opponent process 
The color opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner. The three types of cones have some overlap in the wavelengths of light to which they respond, so it is more efficient for the visual system to record differences between the responses of cones, rather than each type of cone's individual response. The opponent color theory suggests that there are three opponent channels: red versus green, blue versus yellow, and black versus white (the latter type is achromatic and detects light-dark variation, or luminance). Responses to one color of an opponent channel are antagonistic to those to the other color.
Claimed evidence for ability to see impossible colors 
In 1983, Hewitt D. Crane and Thomas P. Piantanida carried out tests using a device that had a field of a vertical red stripe adjacent to a vertical green stripe (or in some cases, yellow–blue). In contrast to apparatus used in simpler tests, the device had the ability to track involuntary eye movement and to adjust mirrors so that the image would appear to be completely stable. The boundary of the red–green stripes was stabilised on the retina of one eye while the other eye was patched and the field outside the stripes was blanked with occluders. This allowed for a mixing of the two colors in the brain, producing neither green for a yellow–blue test, nor brown for a red–green test, but new colors entirely. Some of the volunteers for the experiment even reported that afterwards, they could still imagine the new colors for a period of time.
However, because Hsieh and Tse did not use stabilized images to produce filling in, the colors produce by their procedure have no bearing on the forbidden colors reported by Crane and Piantanida's subjects, some of whom were opponent-color vision scientists. Furthermore, the forbidden colors reported by Crane and Piantanida were replicated in the laboratory of Henrick Gerrits at the University of Nijmegen using a completely different method of retinal image stabilization.
See also 
- Imaginary color (non-physical color, unrealizable color), a point in a color space that correspond to a color perception that cannot be produced by any physical (non-negative) light spectrum.
- False-color image, an image that depicts an object in colors that differ from those a photograph would show.
- Color mixing
- Opponent process
- Bastard (color)
- List of fictional colors
- Crane, Hewitt D.; Piantanida, Thomas P. (1983). "On Seeing Reddish Green and Yellowish Blue". Science 221 (4615): 1078–80. doi:10.1126/science.221.4615.1078. JSTOR 1691544. PMID 17736657.
- Hsieh, P.-J.; Tse, P.U. (2006). "Illusory color mixing upon perceptual fading and filling-in does not result in 'forbidden colors'". Vision Research 46 (14): 2251–8. doi:10.1016/j.visres.2005.11.030. PMID 16469353.
Further reading 
- Billock, Vincent A.; Tsou, Brian H. (2010). "Seeing Forbidden Colors". Scientific American 302 (2): 72–7. doi:10.1038/scientificamerican0210-72. PMID 20128226.
- Takahashi, Shigeko; Ejima, Yoshimichi (1984). "Spatial properties of red-green and yellow-blue perceptual opponent-color response". Vision Research 24 (9): 987–94. doi:10.1016/0042-6989(84)90075-0. PMID 6506487.
- Hibino, H (1992). "Red-green and yellow-blue opponent-color responses as a function of retinal eccentricity". Vision research 32 (10): 1955–64. PMID 1287992.