Why we see in three colors

Color does not exist in the physical world. This statement sounds absurd - surely the sky is blue, grass is green, and stop signs are red? But color is not a property of light itself. It is a sensation created entirely inside your brain, a perceptual interpretation of different wavelengths of electromagnetic radiation. The question of why we see in three colors is really a question about the biological machinery evolution gave us and the clever tricks our brains use to make sense of the light flooding into our eyes.

The electromagnetic spectrum

Light is electromagnetic radiation, waves of oscillating electric and magnetic fields traveling through space at 299,792 kilometers per second. These waves come in an enormous range of wavelengths, from gamma rays shorter than atoms to radio waves longer than buildings. What we call 'visible light' is just a tiny sliver of this spectrum - wavelengths roughly between 380 and 700 nanometers.

Why this particular range? Evolution shaped our eyes to be sensitive to the wavelengths most abundant in sunlight reaching Earth's surface. The sun emits most of its energy in this range, and Earth's atmosphere is relatively transparent to these wavelengths. Eyes that could see radio waves or X-rays would be largely useless - there is simply not enough of that radiation in our environment to form useful images.

The relationship between wavelength and color is not arbitrary - it follows from the physics of how light interacts with matter. Shorter wavelengths carry more energy per photon (this is why ultraviolet can cause sunburn while visible light cannot). When light strikes an object, some wavelengths are absorbed and others reflected, depending on the molecular structure of the material. A leaf appears green because chlorophyll molecules absorb red and blue light for photosynthesis while reflecting green. The color we perceive is quite literally what remains after the object takes what it needs.

Cone cells: the color detectors

The retina at the back of your eye contains two types of photoreceptors: rods and cones. Rods are extremely sensitive to light but cannot distinguish colors - they provide your night vision. Cones are responsible for color vision and work best in bright light. Most humans have three types of cones, each containing a different photopigment that responds most strongly to a particular range of wavelengths.

The S-cones (short wavelength) respond most to blue-violet light around 420 nm. The M-cones (medium wavelength) peak at green light around 530 nm. The L-cones (long wavelength) are most sensitive to yellow-green light around 560 nm - not red, as is often claimed. The perception of red comes from the difference between L and M cone activation, not from a cone specifically tuned to red wavelengths.

Each cone type contains millions of photopigment molecules called opsins, proteins that change shape when they absorb light of the right wavelength. This shape change triggers an electrical signal that travels along the optic nerve to the brain. The three opsin types differ in just a handful of amino acids - tiny genetic variations that shift their peak sensitivity across the spectrum. This is why color blindness often results from just a single gene mutation: change a few amino acids, and the cone's sensitivity curve shifts, overlapping too much with another type.

Trichromacy: three dimensions of color

Three types of cones means three-dimensional color vision - any color we can perceive can be described by three numbers representing the stimulation levels of each cone type. This is called trichromacy, and it is why three-color systems (RGB for displays, CMY for printing) can reproduce such a wide range of colors.

But here is a crucial insight: having only three types of color detectors means we cannot detect the actual spectral composition of light. A rainbow contains a continuous spectrum of wavelengths, but our eyes compress this infinite-dimensional information into just three numbers. Two completely different light spectra can produce identical cone responses and thus appear as the same color - a phenomenon called metamerism.

// Two physically different spectra that appear identical
const spectrum1 = { type: 'monochromatic', wavelength: 580 } // Pure yellow light
const spectrum2 = { type: 'mixture', red: 620, green: 540 }  // Red + green mixture

// Both produce the same cone response:
// L: 0.85, M: 0.75, S: 0.05
// Your brain perceives both as 'yellow'

This is why RGB displays work. A pixel showing 'yellow' is not emitting yellow light - it is emitting red and green light simultaneously. The mixture stimulates your cones in the same pattern as actual yellow light would, so you perceive yellow. It is a trick, but one so effective that you cannot tell the difference.

Color scientists often describe colors in LMS color space, named directly after the three cone types (Long, Medium, Short). Any visible color can be plotted as a point in this three-dimensional space, with coordinates representing how strongly each cone type responds. The LMS representation is fundamental - every other color space (RGB, CMYK, HSL, Lab) is ultimately a mathematical transformation of these basic biological responses.

Opponent processing: how the brain interprets color

The story does not end at the cones. Before color information reaches your conscious awareness, it undergoes significant processing. The signals from the three cone types are combined into opponent channels: red versus green, blue versus yellow, and light versus dark.

This opponent processing explains some curious facts about color perception. You can imagine a reddish-yellow (orange) or a bluish-green (cyan), but you cannot imagine a reddish-green or a yellowish-blue. These combinations are forbidden by the architecture of your visual system - they would require a channel to signal both extremes simultaneously.

The opponent processing happens through specific neural wiring in the retina and lateral geniculate nucleus (LGN). Retinal ganglion cells receive excitatory input from one cone type and inhibitory input from another. A cell that gets excited by L-cones and inhibited by M-cones fires strongly for red light and weakly for green - it literally computes the difference. This neural subtraction enhances color discrimination while reducing the bandwidth needed to transmit color information to the brain.

The evolution of trichromacy

Three cone types is not a universal law - it is just what evolution gave primates. Most mammals are dichromats with only two cone types. Birds and many reptiles are tetrachromats with four, able to see into the ultraviolet. The mantis shrimp, famously, has sixteen types of photoreceptors, though research suggests it does not actually have sixteen-dimensional color vision - it appears to use these channels differently than we use ours.

Why did primates evolve three cones? The leading theory involves diet. The ability to distinguish red from green helps identify ripe fruit against a background of green foliage - a significant advantage for fruit-eating primates. The gene for the L-cone pigment arose from a duplication of the M-cone gene, and the two have been diverging ever since.

The genes for L and M cone opsins sit right next to each other on the X chromosome - a legacy of that ancient duplication event. This proximity makes them vulnerable to errors during DNA replication, which is why red-green color blindness is so much more common than blue-yellow deficiency (the S-cone gene is on chromosome 7, safely isolated). The same genetic instability occasionally works in the opposite direction: some women with unusual X-chromosome combinations may have four cone types, potentially enabling tetrachromatic vision.

Color blindness: when cones malfunction

About 8% of men and 0.5% of women have some form of color vision deficiency. The most common types involve the L and M cones, which are encoded by genes on the X chromosome. Since men have only one X chromosome, a single defective gene causes color blindness; women need defective genes on both X chromosomes.

Color blindness is not simply seeing in grayscale. A person with red-green color blindness still perceives color - they just cannot distinguish certain pairs of colors that appear obviously different to typical observers. A red apple and green leaves might appear similar in hue, distinguishable only by brightness differences.

There are several distinct types of color vision deficiency. Protanopia means missing L-cones entirely - reds appear dark and indistinguishable from certain greens. Deuteranopia means missing M-cones - green sensitivity is absent, but brightness perception remains more normal. Protanomaly and deuteranomaly are milder forms where the cones exist but have shifted sensitivity. Tritanopia, affecting S-cones, is much rarer and causes blue-yellow confusion. Complete achromatopsia - true colorblindness with no functioning cones - is extremely rare and also causes severe light sensitivity.

Implications for digital color

Understanding trichromacy is essential for digital color reproduction. RGB displays work precisely because three primaries are sufficient to stimulate the three cone types in any combination. The specific wavelengths of display primaries are chosen to span a useful gamut while being producible by available technology - they do not match cone peak sensitivities exactly.

This biological foundation also explains the limitations of displays. No RGB display can produce every color humans can see. The colors along the curved edge of the CIE chromaticity diagram - spectral colors from the rainbow - require single wavelengths that cannot be matched by mixing three primaries. The triangle formed by any three primaries always leaves some visible colors outside its bounds.

Designers must consider color vision deficiency when creating interfaces. Red and green should not be the only distinguishing feature between important elements - add shape, position, or text labels. Tools like color blindness simulators help designers see their work as others will. The biology of human color vision is not just academic knowledge; it directly impacts how we should design the digital world.

Color is not in the world; it is in your head. The three-cone system is a biological accident that happens to work well enough for finding ripe fruit and avoiding predators. But understanding this system - its capabilities and limitations - is essential for anyone working with digital color, from display engineers to photographers to interface designers. We all work within the constraints of human trichromacy.