Many of the colors seen in nature, particularly in the plant kingdom, are produced by pigments, which reflect a portion of the light spectrum while absorbing the rest. Green pigments like chlorophyll reflect the green part of the spectrum but absorb the longer red and yellow wavelengths as well as the shorter blue ones. Which specific wavelengths get reflected or absorbed depends on the pigment’s molecular makeup and the exact distances between the atoms in its molecular structures.
Because plants are masters of biochemical synthesis, their cells can concoct many types of pigments, but animals by and large have lost the metabolic pathways to make most of them. Melanin, the predominant pigment in animals, is either brown (eumelanin) or reddish yellow (pheomelanin)—a rather limited palette. To make the richer rainbow of colors they need for decorating and disguising themselves, courting mates and warding off predators, many animals can obtain the needed pigments from their diet. Birds’ bright reds and yellows, for instance, mostly come from carotenoid pigments in their food.
The blue end of the spectrum, however, represents a different challenge because few blue pigments are available to eat in nature. Yet blue jays, neon tetras, poison dart frogs, and many other animals found a solution that doesn’t rely on pigments, evolving optical tricks to make blues (and some greens) a different way. They make what are called “structural colors.”