Impossible colors or forbidden colors are hues that cannot be perceived by 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 pigments (such as paints), or the green color from mixing yellow and blue pigments, but rather colors that appear to be similar to, for example, both red and green, or both yellow and blue. Other colors never experienced by ordinary viewing, but perceivable under special artificial laboratory conditions, would also be termed impossible colors.
Colors that are impossible to achieve through any constant illumination pattern, but simple to access through fatigue effects in the human eye, have been dubbed chimerical. These colors include "Stygian blue" (an impossible deeply saturated blue-black) and "hyperbolic orange" (an orange color that is oversaturated compared to a pure spectral orange).
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: 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.
The opponent process and chimerical colors
Chimerical colors are predicted by the opponent process color theory, which states that color is represented in the human visual system as a separate intensity signal, R/G difference signal, and B/Y difference signal. This yields a 3-D colorspace with orthogonal dimensions of intensity, red/green difference, and blue/yellow difference. Through habituation/fatigue effects in the eye, it is possible to temporarily displace the colorspace signal produced by a particular stimulus. For example, staring at a saturated primary-color field then looking at a white object results in an opposing shift in hue. Through this process, it is possible to produce color signals ("chimerical colors") that cannot be produced in the eye by any steady spectral stimulus. Exploration of the colorspace by this means is major corroborating evidence for the opponent process theory of color vision. Chimerical colors are visible with either monocular or binocular vision and are not observed to reproduce simultaneous qualities of opposing colors (e.g. "yellowish blue").
Claimed evidence for ability to see impossible colors
During 1983, Hewitt D. Crane and Thomas P. Piantanida performed 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 for 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.
- Bastard color
- Color mixing
- False-color image, an image that depicts an object in colors that differ from those a photograph would show.
- 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.
- 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.
- Churchland, Paul (2005). "Chimerical Colors: Some Phenomenological Predictions from Cognitive Neuroscience". Philosophical Psychology 18 (5): 527–560.
- 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.
- 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.