Light is not just something we see — it is the primary energy source for almost all life on Earth. Photosynthesis is the process by which plants, algae, and some bacteria capture photons and convert their energy into chemical energy (sugars), releasing oxygen as a byproduct. In a very real sense, the food you eat and the air you breathe exist because life learned to harvest light.
The key molecule responsible is chlorophyll, which absorbs light most efficiently in the red (~680 nm) and blue (~440 nm) parts of the spectrum. It reflects green light — which is why most plants appear green to us. Chlorophyll evolved to capture the wavelengths where the Sun’s radiation is most intense and most useful for driving chemical reactions. Green light falls in the middle of the visible spectrum where absorption is least efficient for chlorophyll, so it gets bounced back.
Blue is the rarest colour in nature. While the sky and ocean appear blue, these are caused by light scattering, not by blue pigments. True blue pigment is extraordinarily difficult for biological systems to produce. The vast majority of blue in animals — the wings of a Morpho butterfly, the feathers of a blue jay, the rings of a blue-ringed octopus — is structural colour, created not by pigment but by microscopic surface structures that interfere with light waves and selectively reflect blue wavelengths.
Blue flowers are also relatively rare and typically achieve their colour through complex pH manipulation of pigments like anthocyanins. There are very few naturally occurring blue minerals or organisms that use actual blue pigment. This rarity may partly explain why blue has been perceived as special, even magical, across many human cultures.
The human eye is a biological camera, but the real work of seeing colour happens in the retina — a thin layer of nervous tissue lining the back of the eye. The retina contains two main types of photoreceptor cells:
~120 million per eye. Extremely sensitive to light, responsible for vision in dim conditions. They do not distinguish colour — only brightness. This is why everything looks grey in near-darkness.
~6–7 million per eye. Responsible for colour vision. They come in three types, each sensitive to a different range of wavelengths: S-cones (short, ~420 nm — blue), M-cones (medium, ~530 nm — green), and L-cones (long, ~560 nm — red). The brain interprets colour by comparing the relative activation levels of all three cone types.
The chemical that makes this possible is rhodopsin (in rods) and related photopsins (in cones). Rhodopsin is made from a protein called opsin combined with retinal, a molecule derived from vitamin A. When a photon hits a rhodopsin molecule, the retinal changes shape, triggering an electrical signal that travels along the optic nerve to the brain. This is why vitamin A deficiency leads to night blindness — without enough retinal, the rods cannot regenerate rhodopsin efficiently.
How many colours can we actually see? The typical human eye can distinguish roughly 1 million distinct colours. This number comes from the combination of our three cone types: each can discriminate around 100 levels of intensity, and 100 × 100 × 100 gives about 1 million unique combinations. In practice, the actual number varies from person to person depending on the density and health of their cones, lighting conditions, and even training. Under ideal laboratory conditions, some researchers have estimated the upper bound at around 2–3 million distinguishable shades — but in everyday life, 1 million is a reasonable working figure.
Colour vision deficiency (commonly called colour blindness or daltonism, after John Dalton who first described his own condition in 1794) occurs when one or more cone types are absent or function abnormally. The genes for M-cones and L-cones are located on the X chromosome, which is why colour blindness is far more common in men (~8%) than in women (~0.5%) — men have only one X chromosome, so a single defective gene has no backup.
Missing or defective L-cones (red). Red appears dark or brownish. The most disorienting type for everyday life.
Missing or defective M-cones (green). The most common form. Greens and reds are confused, but brightness perception is normal.
Missing or defective S-cones (blue). Very rare. Blues and yellows are confused. Not linked to the X chromosome.
Complete absence of functioning cones. Extremely rare (~1 in 30,000). The world appears entirely in shades of grey.
While most humans are trichromats (three cone types), a small percentage of women may be tetrachromats — possessing four distinct types of cone cells. This is possible because the genes for M and L cones sit on the X chromosome, and women have two X chromosomes. If one X carries a slightly mutated version of a cone gene, it can produce a fourth cone type sensitive to wavelengths between the standard M and L cones.
Research estimates that up to 12% of women may carry the genetic potential for tetrachromacy, though only a small fraction appear to be functional tetrachromats — meaning their brains actually use the fourth cone type to perceive additional colour distinctions invisible to trichromats. Functional tetrachromats can reportedly distinguish between shades that look identical to the rest of us, seeing up to 100 million colours compared to the roughly 1 million a typical trichromat perceives.
Humans are far from having the best colour vision in nature. Different species have evolved visual systems tailored to their ecological niches:
Have 16 types of colour receptors (compared to our 3), including sensitivity to ultraviolet and polarized light. They see a world of colour far beyond our comprehension.
Cannot see red but can see ultraviolet. Flowers that look plain white to us display vivid UV patterns that guide bees to nectar.
Dichromats — they have two cone types and see roughly the equivalent of red-green colour blindness in humans. They distinguish blue and yellow well, but red and green look similar.
Most birds are tetrachromats with a UV-sensitive cone. They see ultraviolet patterns in feathers that are invisible to us, which play a role in mate selection.
Some pit vipers have infrared-sensing pit organs, giving them a "thermal image" of the world overlaid on their visible vision.
Octopuses and cuttlefish are technically colour-blind (one cone type) yet are masters of camouflage. They may perceive colour through their skin or by detecting the polarization of light.
Colour plays crucial roles across species: mate attraction (peacock feathers, bird plumage), warning signals (poison dart frogs, wasps), camouflage (chameleons, cuttlefish), and finding food (bees navigating to flowers). The evolution of colour vision is inseparable from the evolution of colour itself in nature.