Through the Kaleidoscope

Every color you encounter in your surroundings can be discovered in the natural world. Nearly every hue imaginable can be found in plants, animals, or minerals. However, when it comes to identifying the least common colors in nature, two key factors come into play: physics and evolution. Let's begin with the physical aspect. Colors come into existence when wavelengths of light interact with objects. Most of the colors you see outside of screens are produced in one of two primary ways.

In the case of absorption-based colors, certain wavelengths are absorbed by an object, while others are not. This results in a non-reflective, matte final color generated by the remaining light waves. The majority of naturally occurring colors fall into this category, including the colors of many fruits and flowers. Plants, for instance, contain pigments that absorb light as part of photosynthesis, the process through which they convert sunlight into energy. Various plants have evolved different pigments, each responsible for creating different colors. However, pigments tend to more readily absorb higher energy wavelengths than lower energy ones. Blue light, for instance, boasts some of the highest energy wavelengths in the visible spectrum, and numerous pigments have evolved to absorb these blue wavelengths. Even green plants contain pigments like chlorophyll, which absorbs blue and red wavelengths to produce the characteristic green color in nature. While carotenoids, another class of pigments, are present in most green plants, they become visible during the fall when chlorophyll breaks down to conserve energy for the winter. Regardless of whether pigments work independently or in conjunction, they all have the capacity to absorb blue light, which is why almost all plants exhibit this characteristic. Even fruits and flowers that may appear blue have pigments that are actually red or purple, and their blue appearance typically occurs under specific chemical conditions.

So, does this make blue the rarest color in the natural world? Not quite. Absorption is just one of two primary ways that light generates color. In the second method, some wavelengths of light are scattered and amplified, overpowering others to determine the final color of an object. These structural colors arise from the fact that some objects consist of microscopic particles that can create nanostructures interfering with visible light. For instance, a feather may have no blue pigments, but when light hits it, the electrons within its nanostructure vibrate at the same frequency as the light wave, producing a new wave with the same frequency. This initiates a chain reaction, amplifying and scattering blue light. Different shapes and sizes of nanostructures scatter various wavelengths, with blue being the most common structural color because it is typically highly scattered by these nanostructures. Conversely, low-energy wavelengths like red are only weakly scattered. Even when an object evolves specific nanostructures that strongly scatter red light, they resonate with other wavelengths, leading to the object appearing red only from certain angles of illumination and observation.

This leads us to two contenders for the title of nature's rarest color: absorption-based matte blues and structural iridescent reds. Of these, structural reds are much scarcer. Only a few animals and rocks are capable of scattering red light, and none of them scatter red light exclusively. Red and blue, being rare in one way and common in another, are colors that we frequently encounter. Consequently, the color least likely to be generated in both structural and absorption-based forms is violet. Violet should not be confused with purple, which is a combination of red and blue light. Violet occupies only a small portion of the visible light spectrum, and there are very few precise nanostructures capable of exclusively scattering violet light. Furthermore, violet wavelengths are even more energetic than blue ones, making them prone to absorption by pigments. Thus, if you ever come across the iridescent violet wings of a purple emperor butterfly, take a moment to admire one of nature's rarest spectacles.