What Is the Rarest Colour in Nature?

Blue is the rarest colour in nature, and a true blue pigment is rarer still. Fewer than one in ten flowering plants make blue flowers, and almost no animal makes a blue pigment at all. Blue jays, morpho butterflies, peacocks and blue-eyed humans are all producing blue without any blue substance in them: they build microscopic structures that scatter blue light and let the rest pass. The colour is real; the chemistry is a trick of geometry. Violet and true purple are the next rarest, for related reasons. Green, brown and yellow are everywhere.

• Blue is the rarest colour in nature, and a true blue pigment is rarer still.
• Fewer than one in ten flowering plants make blue flowers; almost no animal makes blue pigment.
• Blue jays, morpho butterflies and blue eyes make blue with microscopic structure, not chemistry.
• Violet and true purple are the next rarest; green, brown and yellow are everywhere.

Here is why blue is so hard, and how nature cheats.

Because making a blue pigment is chemically expensive. A pigment looks blue when it absorbs the red and yellow end of the spectrum and reflects the short-wavelength blue. That requires a large, complex molecule with a very specific structure, and most of the pigment families that living things use, the carotenoids that make flamingos pink and the melanins that make skin brown, simply cannot get there. Plants manage blue with a family called anthocyanins, but only under particular conditions of acidity and with metal ions to help, which is why the same anthocyanin can be red in one flower and blue in another. Animals almost never manage it at all.

Structure. A morpho butterfly's wing scales are covered in microscopic ridges spaced at roughly the wavelength of blue light. Light bouncing off the layers interferes with itself, reinforcing blue and cancelling the rest. A blue jay's feathers contain a spongy layer of keratin with air pockets of just the right size to scatter blue. Blue eyes are the same story: there is no blue pigment in a blue iris, only a thin layer that scatters short wavelengths over a dark background. Grind up a blue feather and the blue vanishes, because you have destroyed the structure. Grind up a red one and the red pigment is still there.

This is the same physics as a soap bubble, explained in the soap bubbles article, and the same physics as an iridescent beetle, covered in why butterflies and beetles look iridescent. Nature found interference before Newton did.

Very nearly none, which is what makes the exceptions famous. Two species of butterfly in the genus Nessaea are the best-documented case of a genuine blue pigment in an animal, and a handful of marine creatures manage blue through unusual protein chemistry. Everything else that looks blue, from a kingfisher to a poison dart frog to a mandarin fish, is using structure. If you ever see a claim that a blue bird has blue pigment, it is almost certainly wrong.

Violet and true purple. They sit at the far short-wavelength end of what we can see, and they require either a blue-making structure tuned even finer, or a pigment more complex than blue's. Purple flowers are common because plants can mix red and blue anthocyanins, but a purple animal is a genuine oddity. Black, oddly, is also rarer than it looks: what appears black in nature is often very dark brown melanin, and the true "super black" of some birds of paradise is, once again, a structural trick that traps light in microscopic pits.

Green on land and blue at sea. Green because chlorophyll is everywhere, and blue at sea because water absorbs red and scatters blue, which the ocean colour article covers. Among animals, browns and greys dominate because melanin is cheap to make and hides you. Red, orange and yellow are the next most common, because carotenoids are freely available in food; the flamingo is the famous example of an animal wearing its dinner.

For most of human history, blue pigment was rarer than gold. Ultramarine was ground from lapis lazuli carried from a single region of Afghanistan and cost more than the painter. The history of blue tells that story. We inherited the value long after chemistry made the pigment cheap. A blue that is easy to make is still, somewhere in the back of the mind, a small luxury.

In a translucent colour object. The blues in a CMY Cube are made the way a printer makes blue: cyan and magenta overlapping and removing red and green from white light. It is subtraction rather than structural scattering, but the lesson is identical to the butterfly's. Nothing in the cube is blue. Blue is what is left when the other wavelengths have been taken away. The Mundus, a twelve-faced dodecahedron, throws more of those overlaps at once and is the piece in the range that leans hardest into blues and violets.

Blue is the rarest colour in nature because blue pigment is chemically hard, so almost every blue animal fakes it with microscopic structure that scatters blue light. Violet and true purple come next. Green and brown are everywhere because their chemistry is cheap. And the rarity is why humans have treated blue as precious for thousands of years, long after it stopped being so.

See blue made by subtraction, and view light through a different lens.