Impossible colors

Reddish Green redirects here. Or see Reddish (an area of the Metropolitan Borough of Stockport, in Greater Manchester, England).

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.

Where opposing colors cancel each other out, the remaining color on the vertical axis is perceived. However, under special conditions, a mixture of opposing colors can be seen without the remaining color interfering.

Opponent process

Main article: 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

Some people may be able to see the color "yellow–blue" in this image by allowing their eyes to cross so that both + symbols are on top of each other.

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.

Other researchers dispute the existence of colors forbidden by opponency theory and claim they are, in reality, intermediate colors. See also binocular rivalry.

Impossible colors in fiction

In 1927, American horror fiction author H. P. Lovecraft wrote a short story called "The Colour Out of Space" in which a meteorite crashed into a family farm in rural New England. The meteorite contained a mysterious globule of a color that was "almost impossible to describe," with a note that it was "only by analogy" that professors studying the globule called it a color at all.

David Lindsay in A Voyage to Arcturus described ulfire and jale, two colors visible under the sun Alppain: "Just as blue is delicate and mysterious, yellow clear and unsubtle, and red sanguine and passionate, so he felt ulfire to be wild and painful [and] jale [to be] dreamlike, feverish, and voluptuous."

In 1949, Enid Blyton wrote The Mountain of Adventure, in which the children become involved in an experiment to create weightlessness: "Out of the hole in the pit floor shone a brilliant mass of colour — but a colour the children did not know!"

In 1955, the poet Robert Graves wrote "Welsh Incident," in which something unusual from the sea caves of Criccieth is described as "mostly nameless colours, colours you'd like to see."

Octarine is Terry Pratchett's imaginary eighth color, described as a "greenish-yellow purple."

Mgru is a brand new colour described in a short story of the same name by Stephen Moles as being like "a sarcastic pink or orange, but with a hint of gold impersonating lime", "radioactive claret" and "a really, really aggressive beige".

A hoax or spoof recording by Negativland, featuring the fictional character Crosley Bendix, purports to describe the newly discovered, "fourth primary" color, named "squant."

"hTun" is an impossible color that is "similar to brown" in the book Fairest, by Gail Carson Levine.

In episode 4 of the first series of Nebulous, "Holofile 333: Madness Is a Strange Colour", Vartox Paint Company's new color, Garrow (a sort of yellowy black but with more of a pinky green feel...), is sending people insane.

In "Reincarnation", the final episode of the sixth season of Futurama, the "Colorama" segment ends with Fry accidentally creating an entirely new color in a rainbow. Since the entire segment is in black and white, this new color appears as just another shade of grey to the viewer.

See also

• Color
• Color mixing
• Hue
• Opponent process
• Imaginary color

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.