Impossible color
The world around us explodes with color, a vibrant tapestry woven by light and interpreted by our brains. But what if there are colors, or combinations of colors, that our eyes are simply not designed to see—colors that are truly impossible? From theoretical hues to mind-bending perceptual tricks, the realm of "impossible colors" challenges our very understanding of sight. Our visual system relies on an "opponent process" where colors are perceived in opposing pairs, making mixtures like "reddish-green" normally impossible to see. While some "imaginary colors" are mathematical constructs outside our normal visual range, others, known as "chimerical colors," can be temporarily perceived through specific visual fatigue techniques. Through specialized eye-tracking experiments, scientists have even bypassed the brain's natural color processing, allowing some individuals to genuinely perceive these previously 'forbidden' hues.
AI Summary
The world around us explodes with color, a vibrant tapestry woven by light and interpreted by our brains. But what if there are colors, or combinations of colors, that our eyes are simply not designed to see—colors that are truly impossible? From theoretical hues to mind-bending perceptual tricks, the realm of "impossible colors" challenges our very understanding of sight.
- Our visual system relies on an "opponent process" where colors are perceived in opposing pairs, making mixtures like "reddish-green" normally impossible to see.
- While some "imaginary colors" are mathematical constructs outside our normal visual range, others, known as "chimerical colors," can be temporarily perceived through specific visual fatigue techniques.
- Through specialized eye-tracking experiments, scientists have even bypassed the brain's natural color processing, allowing some individuals to genuinely perceive these previously 'forbidden' hues.
What Makes a Color 'Impossible'?
Normally, our visual system processes an incredible spectrum of colors. But an "impossible color" is one that defies this everyday perception—a hue that, for various reasons, our brains aren't equipped to interpret from ordinary light. These aren't just fictional shades; some arise from the very mechanics of how we see.
The Opponent Process Theory
To understand impossible colors, we first need to grasp how we see color. Our eyes contain three types of cone cells, each sensitive to different wavelengths of light: short (blue), medium (green), and long (red). Instead of simply adding up these signals, our brain processes them in an ingenious way known as the "opponent process."
This theory proposes that our visual system interprets color information by comparing these cone signals in an antagonistic, or opposing, manner. It creates three key "opponent channels" that dictate our perception.
For each channel, the brain can only register one color at a time from a single point on the retina. You can see red, or you can see green, but you can't see both at the same place, at the same time—the signals cancel each other out. This is why you never see a "reddish-green" or a "yellowish-blue" in normal vision.
Red versus Green Blue versus Yellow Black versus White (which dictates light and dark, or luminance)
Imaginary Colors: Beyond the Spectrum of Light
Beyond these perceived impossibilities, there are colors that are truly "imaginary." These are theoretical points within a color space—mathematical descriptions of color—that simply cannot be created by any real, physical light spectrum our eyes could encounter. No physical object, seen under normal conditions, could ever possess such a color.
The reason lies in the overlap of our cone cells' sensitivities. For instance, our medium-wavelength (M) cones, which largely perceive green, always overlap in sensitivity with either our short (S) or long (L) cones, or both. This means that any light strong enough to excite the 'green' M cones will also excite the 'blue' S cones or the 'red' L cones to some degree.
Therefore, no single wavelength of light, nor any combination of wavelengths, can ever exclusively stimulate just the M cones. If it were possible to excite only the M cones, we would perceive an imaginary "hyper-green"—a color greener than any green physically possible in our world.
The Elusive 'Olo'
Intriguingly, recent research has reportedly pushed the boundaries here. By using advanced imaging and a steerable laser to illuminate only the M cones in experimental subjects, scientists claim to have induced the perception of such a "hyper-green." They named this unprecedented blue-green sensation "olo." However, whether "olo" truly represents a new color or merely an extreme saturation of a known hue remains a topic of scientific debate.
Imaginary Colors in Color Spaces
Even though they are beyond normal perception, imaginary colors play a crucial role in the mathematical models that define color spaces. Think of it like this: if you mix any two real colors, you get another real color somewhere along the line between them. Mix three, and you define a triangle of colors—called a "gamut"—that you can create. But the full range of colors we can actually see isn't a perfect triangle, it's more of a curved shape.
To capture the broadest possible range of real colors within a defined system, color scientists often choose "primary colors" that exist outside the real, visible spectrum—these are imaginary primary colors. This allows for a much larger mathematical gamut, even if the primary points themselves are unobservable. Commercial displays, like your TV or phone screen, aim to get as close as possible to these theoretical primaries using real phosphors to achieve the widest possible range of visible colors.
Chimerical Colors: The Afterimage Effect
Another type of "impossible color" is the "chimerical color." These aren't theoretical, but rather temporary visual phenomena we can experience through a clever trick of our own biology. They exploit how our cone cells can become fatigued after staring intensely at a particular color.
When you stare at a strong color for a while, the cone cells responsible for seeing that color become desensitized, or 'fatigued.' Then, when you look away at a neutral surface—like white or black—your other, rested cone cells overcompensate, creating a vivid afterimage in the complementary color. These afterimages can be incredibly intense, pushing beyond normal saturation levels.
Types of Chimerical Colors
Stygian colors: These appear simultaneously dark and impossibly saturated. Imagine staring at bright yellow, then looking at black. The resulting dark blue afterimage seems to be as dark as the black background itself, yet intensely blue—a saturation that normal vision can't achieve against such low light. Self-luminous colors: These mimic the effect of a glowing material, even when viewed on a non-emissive surface like paper. Stare at green, then look at white; the red afterimage can appear brighter than the white background itself, as if glowing. Hyperbolic colors: These are impossibly highly saturated. If you stare at bright cyan, then look at an orange object, the orange afterimage can make the perceived orange even purer and more vibrant than any physical orange light could produce.
Breaking the Opponent Barrier: Seeing "Forbidden" Hues
The most mind-bending "impossible colors" are those that defy the opponent process itself—hues like "reddish-green" or "yellowish-blue." Normally, these are outright forbidden by our visual system because the opposing signals cancel each other out.
However, in 1983, scientists Hewitt Crane and Thomas Piantanida devised an ingenious experiment. They used an eye-tracker to stabilize images on a subject's retina: a vertical red stripe directly adjacent to a vertical green stripe. Because the image moved with the eye, the boundary between the colors remained fixed on the same neurons.
Under these precise conditions, something extraordinary happened. The edges between the stripes seemed to vanish, and the colors bled into each other in the brain's visual cortex. Subjects reported seeing entirely new colors—hues that were not a mix of paint or light, but genuinely novel experiences. Some described a uniform field of a new color; others saw shimmering patterns of red and green dots, or islands of one color on another.
Many volunteers found these new colors indescribable, even those with extensive color vocabularies. They knew they were seeing a color, but it simply defied existing labels—a true testament to perceiving something truly novel. Fascinatingly, some reported being able to mentally recall these new colors for a time after the experiment.
Later experiments in 2001 by Billock, Gleason, and Tsou refined these findings, controlling for factors like the perceived brightness of the colors. They observed subjects reporting transparency phenomena—as if the opponent colors existed in different depth planes, one seen through the other. Crucially, when colors were "equiluminant" (equally bright), subjects explicitly saw "reddish-greens" and "bluish-yellows"—direct evidence of breaking the opponent process.
These groundbreaking experiments suggest a "soft-wired model" of color opponency in the brain, where neural populations compete to fire. By carefully eliminating this competition—in this case, by stabilizing the image—it's possible to allow mutually exclusive neurons to fire simultaneously, revealing colors previously locked away in our perception.
Impossible Colors in Fiction
The allure of colors beyond human perception has long captured the imagination of storytellers. In fiction, these hues often serve as powerful allegories, hinting at hidden realities, alien sensations, or magical abilities. Since they are, by definition, impossible to visualize, authors rely on evocative descriptions to convey their strangeness.
From cosmic horror to whimsical fantasy, these fictional impossible colors remind us that our familiar visual world might just be a small slice of a much grander, unimagined spectrum.
Ambrose Bierce's 1893 short story "The Damned Thing" features a monster that is invisible because it's the color of light beyond human senses. In David Lindsay's 1920 novel "A Voyage to Arcturus," two new primary colors, "ulfire" and "jale," reshape perception. H.P. Lovecraft's iconic 1927 story "The Colour Out of Space" describes an unnamed, unearthly hue generated by alien entities. Terry Pratchett's Discworld series introduces "octarine," the eighth color of the spectrum, described as a "fluorescent greenish-yellow purple" that only wizards and cats can see. In "The Good Place," the immortal being Michael's favorite color is "pleurigloss," beautifully described as "the color of when a soldier comes home from war and sees his dog for the first time."
Article
Impossible color
The human eye's red-to-green and blue-to-yellow values of each one-wavelength visible color
Human color sensation is defined by the sensitivity curves (shown here normalized) of the three kinds of cone cells: respectively the short-, medium- and long-wavelength types.
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.
Opponent process
Impossible color
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 • Black versus white (this is achromatic and detects light–dark variation or luminance)
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.
Imaginary colors
Impossible color
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.
Olo
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.
Imaginary colors in color spaces
Although they cannot be seen in normal vision, imaginary colors are often found in the mathematical descriptions that define color spaces.
Any additive mixture of two real colors is also a real color. When colors are displayed in the CIE 1931 XYZ color space, additive mixture results in color along the line between the colors being mixed. By mixing any three colors, one can therefore create any color contained in the triangle they describe – this is called the gamut formed by those three colors, which are called primary colors. Any colors outside of this triangle cannot be obtained by mixing the chosen primaries.
When defining primaries, the goal is often to leave as many real colors in gamut as possible. Since the region of real colors is not a triangle (see illustration), it is not possible to pick three real colors that span the whole region. The gamut can be increased by selecting more than three real primary colors, but since the region of real colors is bounded by a smooth curve, there will always be some colors near its edges that are left out. For this reason, primary colors are often chosen that are outside of the region of real colors – that is, imaginary or fictitious primary colors – in order to capture the greatest area of real colors.
In computer and television screen color displays, the corners of the gamut triangle are defined by commercially available phosphors chosen to be as near as possible to pure red, green, and blue, within the area of real colors. Because of this, these displays inevitably exhibit colors nearest to real colors lying within its gamut triangle, rather than exact matches to real colors that plot outside of it. The specific gamuts available to commercial display devices vary by manufacturer and model and are often defined as part of international standards – for example, the gamut of chromaticities defined by sRGB color space was developed into a standard (IEC 61966-2-1:1999 ) by the International Electrotechnical Commission.
Chimerical colors
Impossible color
By staring at a "fatigue template" for 20–60 seconds, then switching to a neutral target, it is possible to view "impossible" colors.
A chimerical color is an imaginary color that can be seen temporarily by looking steadily at a strong color until some of the cone cells become fatigued, temporarily changing their color sensitivities, and then looking at a markedly different color. The direct trichromatic description of vision cannot explain these colors, which can involve saturation signals outside the physical gamut imposed by the trichromatic model. Opponent process color theories, which treat intensity and chroma as separate visual signals, provide a biophysical explanation of these chimerical colors. For example, staring at a saturated primary-color field and then looking at a white object results in an opposing shift in hue, causing an afterimage of the complementary color. Exploration of the color space outside the range of "real colors" by this means has been regarded as a major corroborating evidence for the opponent-process theory of color vision, but chimerical colors can also be explained without it. Chimerical colors can be seen while seeing with one eye or with both eyes, and are not observed to reproduce simultaneously qualities of opposing colors (e.g. "yellowish blue"). Chimerical colors include:
Stygian colors: These are simultaneously dark and impossibly saturated. For example, to see "stygian blue": staring at bright yellow causes a dark blue afterimage, then on looking at black, the blue is seen as blue against the black, also as dark as the black. The color is not possible to achieve through normal vision, because the lack of incident light (in the black) prevents saturation of the blue/yellow chromatic signal (the blue appearance). Self-luminous colors: These mimic the effect of glowing material, even when viewed on a medium such as paper, which can only reflect and not emit its own light. For example, to see "self-luminous red": staring at green causes a red afterimage, then on looking at white, the red is seen against the white and may seem to be brighter than the white. Hyperbolic colors: These are impossibly highly saturated. For example, to see "hyperbolic orange": staring at bright cyan causes an orange afterimage, then on looking at orange, the resulting orange afterimage seen against the orange background may cause an orange color purer than the purest orange color that can be made by any normally seen light.
Colors outside physical color space
Impossible color
Some people may be able to see the color "yellow–blue" in this image by letting their eyes cross so that both + symbols are on top of each other. In this image, both RGB and Natural Color System color pairs are provided. It may be necessary to zoom to adjust the image.
Some people may be able to see the color "red-green" in this image by letting their eyes cross so that both + symbols are on top of each other. In this image, both RGB and Natural Color System color pairs are provided. It may be necessary to zoom to adjust the image.
Most people see very bright colored concentric circles in this pattern, if it is printed and rotated at around 150–300 rpm. Alternate version with inverse contrast yields opposite effect.
According to the opponent-process theory, under normal circumstances, there is no hue that could be described as a mixture of opponent hues; that is, as a hue looking "redgreen" or "yellowblue".
In 1983, Hewitt D. Crane and Thomas P. Piantanida performed tests using an eye-tracker device that had a field of a vertical red stripe adjacent to a vertical green stripe, or several narrow alternating red and green stripes (or in some cases, yellow and blue instead). The device could track involuntary movements of one eye (there was a patch over the other eye) and adjust mirrors so the image would follow the eye and the boundaries of the stripes were always on the same places on the eye's retina; the field outside the stripes was blanked with occluders. Under such conditions, the edges between the stripes seemed to disappear (perhaps due to edge-detecting neurons becoming fatigued) and the colors flowed into each other in the brain's visual cortex, overriding the opponency mechanisms and producing not the color expected from mixing paints or from mixing lights on a screen, but new colors entirely, which are not in the CIE 1931 color space, either in its real part or in its imaginary parts. For red-and-green, some saw an even field of the new color; some saw a regular pattern of just-visible green dots and red dots; some saw islands of one color on a background of the other color. Some of the volunteers for the experiment reported that afterward, they could still imagine the new colors for a period of time.
Some observers indicated that although they were aware that what they were viewing was a color (that is, the field was not achromatic), they were unable to name or describe the color. One of these observers was an artist with large color vocabulary. Other observers of the novel hues described the first stimulus as a reddish-green.
In 2001, Vincent A. Billock and Gerald A. Gleason and Brian H. Tsou set up an experiment to test a theory that the 1983 experiment did not control for variations in the perceived luminance of the colors from subject to subject: two colors are equiluminant for an observer when rapidly alternating between the colors produces the least impression of flickering. The 2001 experiment was similar but controlled for luminance. They had these observations:
Some subjects (4 out of 7) described transparency phenomena – as though the opponent colors originated in two depth planes and could be seen, one through the other. ... On occasion (4 out of 7 subjects) the percept was a homogeneous mixture color whose red and green components were as clear and as compelling as the red and blue components of a purple. This percept tended to last longer than the gradient phenomenon. We found that when colors were equiluminant, subjects saw reddish greens, bluish yellows, or a multistable spatial color exchange (an entirely novel perceptual phenomena [sic]); when the colors were nonequiluminant, subjects saw spurious pattern formation.
This led them to propose a "soft-wired model of cortical color opponency", in which populations of neurons compete to fire and in which the "losing" neurons go completely silent. In this model, eliminating competition by, for instance, inhibiting connections between neural populations can allow mutually exclusive neurons to fire together.
Binocular rivalry
Hsieh and Tse in 2006 disputed the existence of colors forbidden by opponency theory and claimed they are, in reality, intermediate colors. However, by their own account their methods differed from Crane and Piantanida: "They stabilized the border between two colors on the retina using an eye tracker linked to deflector mirrors, whereas we relied on visual fixation." Hsieh and Tse do not compare their methods to Billock and Tsou, and do not cite their work, even though it was published five years earlier in 2001.
In fiction
Impossible color
Some works of fiction have mentioned fictional colors outside of the normal human visual spectrum that have not been observed yet and whose observation may require advanced technology, different physics, or magic. Introduction of a new color is often an allegory intending to deliver additional information to the reader. Such colors are primarily discussed in literary works, as they are currently impossible to visualize (when a new color is shown in the episode "Reincarnation" of the animated show Futurama, the animation for that segment of the show is purposely kept in shades of gray).
• One of the earliest examples of fictional colors comes from the Ambrose Bierce 1893 horror short story "The Damned Thing", wherein the titular monster is theorized to have been a color beyond human senses, rendering the monster itself invisible. • The 1920 science fiction novel A Voyage to Arcturus by David Lindsay, mentions two new primary colors, "ulfire" and "jale". • "The Colour Out of Space," a 1927 story by H.P. Lovecraft, is named after an otherwise unnamed color, usually not observable by humans, generated by alien entities. • Marion Zimmer Bradley in her novel The Colors of Space (1963) mentions "the eighth color" made visible during the FTL travel. • Philip K. Dick's 1969 novel Galactic Pot-Healer mentions a color "rej". • Brazilian writer Ziraldo's 1969 children's book Flicts tells the story of a color of the same name (represented as an earthy shade of beige) that is segregated by the other colors found in the rainbow, flags and elsewhere, because flicts is rare, seen as uncharacteristic, and therefore undervalued; at the end of the book, flicts finds its place as the color of the moon. (After being gifted an English copy of the book, Neil Armstrong signed it and wrote "The moon is flicts.") • Garry Kilworth's 1982 story "Blind Windows" involves a search for a fourth primary color. • Terry Pratchett, in his Discworld series that began with The Colour of Magic (1983), describes the eighth color "octarine", resembling a "fluorescent greenish-yellow purple" color, which can be seen only by wizards and cats. • Vernor Vinge's science fiction novel A Deepness in the Sky includes a species who can see a color whose name is translated as "plaid" (including a reference to "alpha plaid"). • In Fallen London (2009), Sunless Sea (2015), and Sunless Skies (2019), which take place in a shared universe created by Failbetter Games, there exist seven colors as part of a "Neathbow" that cannot be viewed in plain sunlight, are counterparts to regular colors, and have fantastical properties, such as "irrigo" and "violant", which remove and reinforce memories, respectively. • "Pleurigloss" is the favorite color of the immortal afterlife-being Michael from the 2020 television show The Good Place. In the show, pleurigloss is described as "the color of when a soldier comes home from war and sees his dog for the first time".