What is the rarest color in nature? - Victoria Hwang

Every color you see in front of you can be found in nature. Some plant, animal, or mineral bears almost every hue imaginable. But which of these colors are you least likely to see in the natural world? There are two factors that drive the rarity of color in nature: physics and evolution.

Let’s start with physics. Colors are generated when wavelengths of light interact with objects, and most of the colors you’ve seen outside a screen were produced in one of two ways. In absorption-based colors, certain wavelengths are absorbed by an object, while others are not. The result is a matte final color generated by these leftover light waves. Most naturally occurring colors fall into this category, including those of many fruits and flowers.

Plants are full of compounds called pigments that absorb light waves as part of photosynthesis, the process by which they convert sunlight into energy. While different plants have evolved different pigments that result in different colors, higher energy wavelengths are more easily absorbed than lower energy ones. And blue light has some of the highest energy wavelengths in the visible spectrum.

Numerous pigments have evolved to absorb blue light, including chlorophyll, which absorbs blue and red wavelengths to produce nature’s trademark green. However, green light is still fairly energetic, and the most common class of pigments evolved to absorb these wavelengths as well. There are over 1,100 types of carotenoids, pigments which absorb high energy blue and green light, while leaving behind the lower energy red and yellow light.

While carotenoids are present in most green plants, they only become visible each fall when chlorophyll gets broken down to save energy for the winter. But whether they’re working alone or side by side, these pigments absorb blue light in virtually all plants. Even fruits and flowers that appear blue actually have pigments that are red or purple, and only truly turn blue under specific chemical conditions.

So, is blue the rarest color in nature? Not quite. Absorption is just one of the two main ways light generates color. In the second method, some wavelengths are scattered and amplified— overpowering the others to determine an object's final color. These structural colors occur because some objects around us are made of microscopic particles which can form nanostructures that interfere with visible light.

For example, this feather has no blue pigments in it. But when light strikes it, the electrons within its nanostructure vibrate at the same frequency as the wave. This makes the particles send out a new wave with the same frequency, starting a chain reaction that amplifies and scatters blue light. Nanostructures of various shapes and sizes scatter different wavelengths, but they typically scatter high-energy wavelengths most easily— making blue the most common structural color.

Meanwhile, low-energy wavelengths like red are only weakly scattered. Even when something evolves specific nanostructures that strongly scatter red light they still resonate with other wavelengths, only appearing red at some angles of illumination and observation. This gives us two contenders for nature’s rarest color: absorption-based matte blues and structural iridescent reds.

Between these two, structural reds are much rarer. Only a handful of animals and rocks scatter red light and none of them scatter red light exclusively. But since red and blue are rare in one way and common in another, we actually end up seeing both colors quite often. So what color is least likely to be generated in structural and absorption-based forms?

The answer is violet. Not to be confused with purple, which is just a combination of red and blue light, violet occupies a small portion of the visible light spectrum. There are only a few nanostructures precise enough to exclusively scatter violet light. And violet wavelengths are even more energetic than blue ones, making them likely to be absorbed by pigment.

So if you ever stumble onto the iridescent violet wings of a purple emperor butterfly, take a second to appreciate one of nature’s rarest spectacles.