Impossible color

Impossible colors are colors that do not appear in ordinary visual functioning. Different color theories suggest different hypothetical colors that humans are incapable of perceiving for one reason or another, and fictional colors are routinely created in popular culture. While some such colors have no basis in reality, phenomena such as cone cell fatigue enable colors to be perceived in certain circumstances that would not be otherwise.

The color opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cone and rod cells in an antagonistic manner. The three types of cone cells 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:

Responses to one color of an opponent channel are antagonistic to those of the other color, and signals output from a place on the retina can contain one or the other but not both, for each opponent pair.

A fictitious color or imaginary color is a point in a color space that corresponds to combinations of cone cell responses in one eye that cannot be produced by the eye in normal circumstances seeing any possible light spectrum. No physical object, perceived by the normal process of vision, can have an imaginary color.

The spectral sensitivity curve of medium-wavelength (M) cone cells overlaps those of short-wavelength (S) and long-wavelength (L) cone cells. Light of any wavelength that interacts with M cones also interacts with S or L cones, or both, to some extent. Therefore, no wavelength and no spectral power distribution excites only the M cones.

A physically realizable stimulus can, unlike the case with the M cones, excite only the L or only the S cones. This can be done using bright lights whose wavelength lies at the very extremes of the visible spectrum. A lightsource that emits light with a wavelength of around 800 nm will exclusively excite the L cones. A lightsource that emits light with a wavelength of around 360 nm will exclusively excite the S cones. As one of the extremes is approached, the signal becomes purer and purer.

If M cones were excited alone, an imaginary color greener than any physically possible green would be perceived. Such a "hyper-green" falls, on the CIE 1931 xy chromaticity diagram and according to CIE 2006 LMS, on the xy coordinates (1.3267164, -0.3267164) (below and to the right of the visible gamut on the diagram). In April 2025, a research group reported achieving exactly this, by using an imaging system to scan the retina and a steerable laser source to illuminate M cones exclusively. The color perceived by experimental subjects matched the predicted sensation, describing the color as a blue-green of unprecedented saturation. It was named "olo", after its coordinates (0, 1, 0) in LMS color space. However, there is some disagreement as to whether olo is really a new color. Approximations to olo may be seen by the opponent-fatigue process, as demonstrated by other hypersaturated colors such as hyperbolic orange, described under "Chimerical Colors" below.

Although they cannot be seen in normal vision, imaginary colors are often found in the mathematical descriptions that define color spaces.