Try to picture reddish-green. Not a muddy brown, not olive, not some blend that sits between the two on a paint chip. An actual color that is simultaneously, unmistakably red and green at once, the way purple is simultaneously red and blue. Most people can’t picture it, and there’s a good biological reason why: color scientists have long considered reddish-green and its cousin, yellowish-blue, to be genuinely impossible for a normal human eye to perceive. And yet, under some very specific and slightly strange laboratory conditions, people have reported seeing exactly that.
Why Certain Color Combinations Are Considered Impossible
The explanation starts with a model of color vision proposed by physiologist Ewald Hering back in 1872, known as opponent process theory. Rather than treating every color as its own independent signal, the theory holds that human color vision runs through three paired channels: red versus green, blue versus yellow, and black versus white. Each channel can lean toward one end or the other, but not both at the same time, the same way a light switch can be on or off but not simultaneously both. This is why you never see brownish-blue described as fundamentally “reddish-green” the way you’d describe purple as reddish-blue. The channel that would need to signal both red and green at once simply isn’t built to do that.
What “Impossible” Actually Means Here
This isn’t just a quirk of language or a failure of imagination. Opponent process theory is one of the most well-supported models in visual science, backed by decades of measurements showing that specific neurons really do respond to red and green, or blue and yellow, in an antagonistic, either-or fashion. For most of the 20th century, that made reddish-green and yellowish-blue seem like a genuinely closed case: not colors that were merely rare or hard to describe, but colors that the human visual system was structurally incapable of producing at all.
The Lab Experiment That Broke the Rules
In 1983, two researchers at SRI International, Hewitt Crane and Thomas Piantanida, published a paper in the journal Science with a genuinely startling claim: under the right conditions, people actually could perceive reddish-green and yellowish-blue. Their method involved showing subjects an image made of two touching vertical stripes, one red and one green in one version, one blue and one yellow in another. Using an eye tracker, they continuously adjusted the image in real time to cancel out the subject’s small, constant eye movements, effectively locking the boundary between the two color stripes onto the exact same patch of retina without interruption.
Why Scientists Mostly Ignored the Finding for Years
The result was strange enough that Crane and Piantanida’s subjects reportedly struggled to describe what they were seeing, since ordinary color vocabulary simply doesn’t have words for it. As the stabilized boundary between the stripes seemed to dissolve, some subjects described a genuinely new, unified color occupying the space where red and green, or blue and yellow, used to meet. Despite being published in a major scientific journal, the finding was met with a strange kind of scientific shrug for years afterward. One vision researcher later described it as being treated like “the crazy old aunt in the attic of vision,” a curiosity nobody quite wanted to engage with directly. It took follow-up studies, including further retinal-stabilization work in the early 2000s and a 2006 study asking participants to match the color they perceived against a computer-based color map, to build a more solid case that something real, not just an artifact of a strange experimental setup, was happening.
The Impossible Colors You Can Actually Try at Home
Crane and Piantanida’s exact setup required specialized eye-tracking equipment most people don’t have lying around, but there’s a related family of visual effects, called chimerical colors, that rely on a much more familiar trick: cone fatigue, the same mechanism behind ordinary afterimages. Stare at a strongly saturated color for long enough and the cone cells responsible for detecting it temporarily tire out, throwing off the balance of signals your visual system expects. Depending on exactly what you look at next, this fatigue can produce colors that feel genuinely stranger than an average afterimage.
Three Different Flavors of “Impossible”
Researchers generally group these into three categories. Stygian colors appear simultaneously as fully saturated color and as dark as black, a genuinely paradoxical-feeling combination best demonstrated with stygian blue: stare at a solid patch of yellow for around 25 seconds to fatigue your red and green sensitive cones, then shift your gaze to a black surface. Since your blue-sensitive cones weren’t fatigued, they fire freely against the dark background, producing a blue that somehow also reads as black at the same time. Self-luminous colors, by contrast, appear to glow with their own inner light even though nothing is actually emitting any. Hyperbolic colors push saturation further than should be visually possible, producing something like an impossibly intense orange or green that feels more vivid than any pigment could actually achieve. None of these are hallucinations in any concerning sense. They’re temporary, harmless perceptual quirks that reveal something real about how color processing works, and they disappear within moments once your cones recover.
Why the Brain Is Willing to Bend Its Own Rules
What both the lab-grade forbidden colors and the more accessible chimerical colors point toward is the same underlying idea: color, as you experience it, isn’t a direct readout of the physical world. It’s a construction, built by neural circuitry that follows a specific, learned set of rules about how signals from red, green, and blue-sensitive cones are supposed to combine. Under normal daily conditions, those rules hold up perfectly well and there’s no reason to ever question them. But push the system into an unusual corner, whether through careful retinal stabilization in a lab or a simple 25-second staring exercise at home, and the rules turn out to be a little more negotiable than a stable, seemingly permanent feature of vision would suggest. The reddish-green you technically aren’t supposed to be able to see isn’t proof that anything is broken. It’s a reminder that what you call “seeing color” was never really about the light itself, so much as about the specific, occasionally bendable interpretation your visual system builds around it.
Frequently Asked Questions
What are “impossible colors” like reddish-green?
Impossible or forbidden colors are combinations like reddish-green and yellowish-blue that opponent process theory predicts a human eye cannot perceive, since these color pairs are processed through the same antagonistic neural channel. Research has shown that under specific laboratory conditions, some people can perceive these combinations anyway.
What was the 1983 experiment that showed people forbidden colors?
Researchers Hewitt Crane and Thomas Piantanida used an eye tracker to keep a red-green or blue-yellow striped image locked onto the same patch of a subject’s retina despite normal eye movements. Subjects reported the boundary between the colors dissolving into a new, hard-to-describe color that combined both hues at once.
Can I see impossible colors without special lab equipment?
Yes, a related category called chimerical colors, including stygian, self-luminous, and hyperbolic colors, can be produced at home using simple cone fatigue tricks, such as staring at a saturated yellow patch and then shifting your gaze to a black surface to see a color that appears simultaneously blue and black.
Are impossible colors harmful to see?
No, these effects are temporary and harmless. They result from ordinary, well-understood processes like cone fatigue and neural adaptation, and normal color perception returns within moments after the effect fades.
