Color perception is a fascinating and complex topic. While most people can see the full spectrum of visible light, comprising around 10 million discernible shades, there are some hues that seemingly do not exist. Known as forbidden colors, impossible colors or imaginary colors, these elusive tints have intrigued scientists and artists for over a century.
What causes forbidden colors?
To understand forbidden colors, we must first consider how we see color. Human color vision relies on cells in the eye called cones, which respond to different wavelengths of light. There are three types of cones:
- S cones detect short wavelengths, which we perceive as blues.
- M cones detect medium wavelengths, which we see as greens.
- L cones pick up longer wavelengths, registering as reds.
When light enters our eye, it stimulates the cones to varying degrees. The balance of activation across the three cone types gives rise to all the colors we can experience. For example, yellow light strongly activates both L and M cones, while purple excites the S and L cones.
The key to forbidden colors lies in how the cones are wired up in the retina. Each cone does not connect directly with the optic nerve that carries signals to the brain. Rather, the cones feed into special intermediary cells, called retinal ganglion cells.
There are three types of retinal ganglion cells:
- +L-M cells that compare L and M cone signals
- -L+M cells that contrast L and M signals
- S-cone cells fed only by S cones
This wiring means certain combinations of cone activity do not correspond to any retinal ganglion cell response. These are the forbidden colors.
Examples of forbidden colors
Various thought experiments can help us imagine forbidden colors that we are unable to see:
Super-purple
Consider an imaginary color that is even more purple than violet light. This “super-purple” would require very strong stimulation of S cones combined with significant activation of L cones. But there are no retinal ganglion cells that can signal this combination, so our brain could never perceive super-purple.
Hyperbolic orange
Now imagine a color resembling orange but with much greater stimulation of L and M cones than is possible with real orange light. This hyperbolic orange would strongly activate both L and M cones, beyond the range that retinal ganglion cells can accommodate. Consequently, it remains out of sight.
Red-green blend
Other forbidden colors involve the simultaneous activation of cones that normally signal opposing colors. For example, shining red light and green light onto the same spot can never mix to show the forbidden red-green color that excites L and M cones equally.
Who discovered forbidden colors?
The concept of forbidden colors emerged in the 19th century when scientists were developing the trichromatic theory of color vision, recognizing that just three cone types underlie our rich color sense. In 1892, the German physiologist Ewald Hering proposed six primary colors – red, green, blue, yellow, white and black – and speculated that combinations of these primaries outside our normal experience might yield unseeable, forbidden colors.
A few decades later, the mathematician and artist Ogden Rood hypothesized that colors indistinguishable from gray might exist. He wrote of “a gray which results from the mixture of complementary colors, and is nothing else but the intermediate tint corresponding to the most saturated pink, yellow, and greenish blue”.
In the 1950s, the idea of forbidden colors gained further attention when researchers started measuring cone responses. Neuroscientist Leo Hurvich and psychologist Dorothea Jameson proposed forbidden colors resulting from cone activity beyond normal limits.
Can we see forbidden colors?
While forbidden colors remain theoretical, there are some ways we can attempt to experience their unusual appearance:
Afterimages
Staring briefly at a colored shape, then looking at a gray background, produces an afterimage in complementary colors. Focusing on red and green shapes can generate a muddy brown forbidden color afterimage.
Retinal fatigue
Exhausting a single cone type, by prolonged exposure to one color, shifts color appearance. For example, after staring at a green cross for 30 seconds, a red cross can take on a forbidden reddish-green hue.
opposing colors
Presenting alternating red and green fields to different areas of the retina rapidly can mix their signals to give brief glimpses of forbidden colors.
LCD displays
Modern LCD screens project independent red, green and blue signals to each pixel. By displaying combinations unseen in nature, they can render vivid forbidden colors.
Applications of impossible colors
While we cannot fully perceive forbidden colors, scientists have explored ways to make use of them:
Probing vision
Studying how we see hypothesized forbidden colors, such as through afterimages, provides insight into the cones, retinal wiring and brain processing that underlies color vision.
Color gamuts
Expanding computer and television displays beyond viewable colors enlarges their color gamut, affording more realistic image rendering.
Camouflage
Since impossible colors do not visibly appear in nature, incorporating them into military camouflage could conceal soldiers from color-sensitive detection.
Anti-counterfeiting
Bank notes and official documents could integrate patches of forbidden color that are hard to replicate as a deterrent to counterfeiters.
| Forbidden color | Cone stimulation | Appearance |
|---|---|---|
| Super-purple | Very strong S, moderate L | More purple than violet |
| Hyperbolic orange | Very strong L and M | Exaggerated orange |
| Red-green | Equal L and M | Impossible to visualize |
Challenges in defining forbidden colors
While forbidden colors provide intriguing thought experiments, some challenges remain in precisely defining them:
Variation in cone responses
The exact cone stimulation limits that bound visible colors differ between individuals, making forbidden boundaries indefinite.
Cortical processing
Cortical neurons in the brain contribute additional processing after the retina, complicating perceptions of hypothetically impossible colors.
Language limits
Since we cannot see forbidden colors, describing them with language is speculative and often subjective.
Different visual contexts
The appearance of imaginary colors would vary based on interactions with adjacent hues, further confounding definitions.
Conclusion
The quest to understand forbidden colors provides insight into the wonders and limitations of human vision while also offering opportunities to advance display technology, camouflage and anti-counterfeiting. After over a century of speculation, these elusive, imaginary hues continue to intrigue us with their seemingly unattainable appearances. While forbidden colors remain confined to thought experiments, they represent a fascinating boundary between the colors we can see and our unfulfilled color experience.