While the natural world is bursting with colour, some hues are much rarer than others. This is because of a combination of two factors: evolution and physics. Evolution selects colours that help animals and plants survive and reproduce. But evolution cannot work alone – it functions in partnership with light’s physical properties.
In the natural world, colour is either produced through absorption-based pigments or structural colour. Pigments absorb specific wavelengths of light and reflect others. We perceive different wavelengths of light as different colours. The wavelengths that pigments do not absorb are the colours we see. For example, if a pigment absorbs high-energy wavelengths, such as blue, but reflects low-energy ones, such as red, we will see that pigment as red.
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Photosynthesis needs high-energy blue light (among other wavelengths) and plants have consequently evolved pigments that absorb, rather than reflect, blue light. The colour blue is therefore quite an unusual sight in the plant world. And because many animals get their pigment through the food-chain, with plants at the bottom of this, blue is relatively rare in animals.
Unlike pigment-based colours, structural colours are formed by colourless molecular structures that scatter light. This scattering creates an iridescent effect where a range of hues can be observed from different angles (such as the feathers of a magpie or the stunning wings of a blue morpho butterfly).
Colour-scattering structures usually scatter high-energy wavelengths. These are more easily scattered than low-energy ones. So while blue is a rare pigment in nature because high-energy blue light is preferentially absorbed by plants, the same physical property also makes blue one of the most common structural colours. Meanwhile, red – a common pigment – is a rare structural colour because it’s low energy, and this light is difficult to scatter.
So considering both pigment and structural colour, what is the rarest colour in nature overall? The answer is violet. The highest-energy wavelength of visible light, violet is very rare as both a pigment and a structural colour. Mammals are unable to create pigments for violet, so you’ll only find it in birds, insects and marine species that can display it through structural colour. Violet birds include the purple honeycreeper, violet-backed starling and imperial amazon (a parrot species). When it comes to insects, you’ll find purple emperor butterflies, and in the ocean, purple sea stars and purple sea slugs.
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Top image: blue morpho butterfly. Credit: Getty
