Forbidden colors are color experiences your brain normally prevents, like a red that is simultaneously green, or a yellow that is simultaneously blue. You can’t see them by mixing paint or adjusting a screen, but a few visual tricks can push your perception past its usual limits. Some methods require nothing more than a colored image and patience, while the most dramatic results come from specialized lab equipment.
Why Your Brain Blocks Certain Colors
Your eyes have three types of color-sensing cells called cones. Two of these types, the ones sensitive to red and green wavelengths, have spectral sensitivities that overlap heavily. They respond to many of the same wavelengths of light, just with slightly different peaks around 530 and 560 nanometers. Your brain processes color by comparing signals from these cone types in an opponent system: red versus green on one channel, blue versus yellow on another. A signal can slide toward red or toward green, but it can’t go both directions at once. That’s why “reddish green” sounds like nonsense. It’s not a limitation of light or pigments. It’s a limitation of how your neural wiring interprets cone signals.
This opponent processing is also why even people who carry a genetic variant for a fourth cone type between the red and green peaks don’t gain a new dimension of color vision. Research published in the Journal of Vision found that when a fourth cone type with sensitivity between the existing red and green cones was modeled, the brain couldn’t learn to wire it separately because natural scenes produce responses from those cones that are too similar to tell apart.
The Stabilized Image Method
The most direct approach to seeing a truly forbidden color comes from a 1983 experiment. Researchers placed subjects in front of adjacent stripes of red and green (or yellow and blue) and used an eye tracker to stabilize the boundary between the stripes on the retina. Normally, your eyes make tiny involuntary movements called microsaccades that keep edges sharp. When the boundary between two colors is locked in place on your retina, something unusual happens: the border dissolves, and the two colors flood into each other.
Subjects reported seeing the entire region as simultaneously red and green, or simultaneously yellow and blue. These weren’t brownish or grayish blends. They were described as colors people had never seen before, colors that genuinely seemed to be both hues at once. The outer edges of the stripes were not stabilized, so they remained visible and kept the overall image from fading entirely.
The catch is that this requires a retinal stabilization rig, which is specialized lab equipment that tracks your eye movements in real time and shifts the projected image to compensate. You can’t fully replicate this at home. However, a rough approximation exists: hold a red and green image (or blue and yellow) very close to your face, cross your eyes slightly so the two colored fields overlap, and hold your gaze as steady as possible. Some people report brief flashes of an unusual mixed percept before the brain reasserts its normal opponent processing. Results vary widely, and most people see muddy olive or brown rather than anything truly forbidden.
Chimerical Colors You Can See Right Now
A more accessible category of impossible colors is called chimerical colors. These exploit afterimages, the lingering color impression your cones produce after staring at a bright hue. Chimerical colors come in three varieties, and you can try all of them with nothing more than a screen or a sheet of colored paper.
Stygian Colors
Stygian colors appear simultaneously dark and intensely saturated, which shouldn’t be possible since dark colors are normally desaturated. To see stygian blue: stare at a bright yellow patch for 20 to 30 seconds, then shift your gaze to a black surface. You’ll see a blue afterimage against the black. The strange part is that the blue appears as dark as the surrounding black while still reading as vividly blue. Your brain is combining “dark as black” with “saturated blue,” a pairing that no real surface or light can produce.
Self-Luminous Colors
These make a flat surface appear to glow as if lit from within. Stare at a green patch for 20 to 30 seconds, then look at a white surface. You’ll see a red afterimage that can appear brighter than the white paper or screen behind it. Paper can only reflect light, never emit it, so a color that looks brighter than white on paper is perceptually impossible. The effect works because the afterimage adds to the lightness of the white background, pushing the perceived brightness past the surface’s actual reflectance.
Hyperbolic Colors
These are colors more saturated than any version of that hue you’ve ever seen from real light. Stare at a bright cyan patch for 20 to 30 seconds, then look at an orange surface. The orange afterimage stacks on top of the real orange, producing an orange so pure it exceeds anything a monitor, printer, or even a laser pointer of that wavelength could produce under normal viewing. The result is a color that feels almost electric, more “orange” than orange has any right to be.
For all three types, a few tips improve results. Use the brightest, most saturated source color you can find. Keep your eyes fixed on one point during the adaptation phase; don’t let them wander. A well-lit room helps for self-luminous colors, while a truly dark environment helps for stygian colors. The effect lasts only a few seconds, so pay attention the instant you shift your gaze.
Lab Technology That Creates New Colors
Researchers at UC Berkeley built an optical imaging platform called Oz that takes a fundamentally different approach. Instead of tricking opponent processing through afterimages or stabilized boundaries, it shines a laser directly into the eye to stimulate only one type of cone at a time. Under normal conditions, any wavelength of light hitting your retina activates multiple cone types to varying degrees. Isolating a single cone type produces a signal the brain has never received from natural light.
Using this method, the team produced a hypersaturated green they named “olo.” Subjects described it as a green unlike anything they had encountered, more vivid than any green surface or screen could display. The technology was developed primarily for diagnosing and understanding eye diseases, but its ability to generate perceptions outside the normal color gamut suggests it could eventually expand what humans can experience visually.
This isn’t something you can try at home, and pointing lasers into your eyes without clinical-grade equipment would be dangerous. But it confirms something the afterimage tricks hint at: the brain’s color processing has more range than everyday light ever asks it to use.
What You’ll Actually Experience
If you try the chimerical color methods above, expect the effect to be brief and somewhat fragile. Most people see it clearly for two to five seconds before it fades. The stygian and self-luminous varieties tend to be the most dramatic because the impossibility is easiest to notice: something that’s both pitch-dark and vividly colored, or something on paper that appears to glow. Hyperbolic colors can be harder to appreciate because the difference between “very saturated orange” and “impossibly saturated orange” is subtle without a side-by-side comparison.
The stabilized-image forbidden colors, the true reddish-green or yellowish-blue percepts, remain largely confined to the lab. Cross-eyed free-fusion attempts work for some people but produce inconsistent results. If you want to try, search for “red green forbidden color” images designed for cross-eye fusion, overlap the fields, and hold your gaze completely still. Even a partial glimpse of the effect is worth the attempt, because the sensation of seeing a color that doesn’t map to anything in your experience is genuinely disorienting in a way that’s hard to describe until you’ve felt it.