Purple: Nature’s Rarest Color

From the fiery red of a sunset to the serene blue of a tranquil sea, it is hard to imagine a world void of the mesmerizing hues displayed in every corner of our natural world. However, within this vibrant spectrum, one color appears to evade our sight much more than others. Purple, often termed the rarest color in nature, has historically been associated with royalty and regality, emblemizing its significance and scarcity. However, the reason for purple’s rarity holds a much deeper scientific intrigue and gives us a window into the ecological complexity of color.

Humans can see color through special cells in our eyes, which react to various wavelengths of light and send this information through the optic nerve. These cells, known as cones, allow the brain to distinguish between various colors. However, purple in itself does not exist as a wavelength of light on the visible light spectrum, a model relating electromagnetic waves to light that humans can view. On the visible light spectrum, shorter wavelengths of light are associated with higher energy, and longer wavelengths of light are associated with lower energy. Purple is not found on this spectrum, as it is merely the combination of red and blue light that allows us to see shades of purple color. Thus, the rarest color and the rarest color of light that humans can see are distinct. The rarest color as a wavelength of light is violet, exhibiting the shortest wavelength on the visible light spectrum and also being very similar to purple. Still, in a discussion of colors that can be viewed by humans, purple remains the rarest.

The combination of its composite colors, namely red and blue, is the reason why purple is so rare. The main mechanism for this is due to how blue and red light are perceived in nature. Certain colors are only visible if they are reflected away from the object being viewed. The two main ways that this occurs are through the absorption of color or the scattering and amplification of wavelengths. 

In absorption-based color, objects absorb certain wavelengths of light while others are reflected away. These reflected wavelengths are then able to travel into your eye where your brain can observe that color. In this form of color display, higher energy light, or light that has a shorter wavelength such as violets and blues, is more easily absorbed in naturally occurring objects such as flowers. This occurs as a result of one main factor: carotenoids, a class of pigments commonly found in plants that express red, yellow, and other low-energy light. Sunflowers, for example, have carotenoids that give them their yellow/orange colors. This means that shorter, higher-energy wavelengths of light are absorbed, including blue, which only has a slightly shorter wavelength than violets and indigos. Since natural organisms primarily represent their colors through an absorption-based technique, blue light is among the rarest in nature. 

On the other hand, some organisms have a unique nanostructure in which the microparticles of these structures can interact with certain wavelengths of light. For example, the blue tarantula's hairs exhibit a specific nanostructure resembling flowers, allowing for the refraction of blue light across multiple angles. This is an example of amplification-based color, where light strikes electrons, which subsequently vibrate at the same frequency as the light wave, amplifying a certain color without the use of pigments. In some organisms, such as the bluebird, there are no pigments that express blue, but they still appear blue as a result of this amplification. In fact, almost all creatures in the animal kingdom are unable to produce any form of blue pigment. However, this form of color display allows for blue coloration to happen more frequently, typically expressing higher energy wavelengths of light over lower energy ones. This means that blues are more likely visible than reds in organisms that have these structures. 

From these two forms of color expression, it is clear that absorption-based color makes red light more common while amplification-based color makes blue light more common. However, this doesn’t answer why purple is so rare if reds and blues are not rare themselves. The problem lies in being able to reflect both red and blue light at the same time, which very few pigments or structures can do. In addition, one of the primary pigments found in plants, chlorophyll, is extremely necessary for life but is also very effective at absorbing reds and blues while reflecting green light. This makes expressing purple incredibly difficult, and only certain organisms are able to perform this task through the use of anthocyanins, a specific class of pigments that can effectively express purple coloration by absorbing greens and reflecting reds and blues. Eggplants, for example, exhibit these anthocyanins in their skin, giving them their purple color. In addition, mammals are physically incapable of producing pigments for blue or purple, emphasizing purple’s rarity. This is because mammals mainly utilize melanins for skin coloration and thus lack the mechanisms to produce the pigments for blue and purple colors.

As a result, purple is typically only visible on birds, fish, and a small selection of plants. Some examples of these include the ochre sea star, the purple grenadier, and the lilac flower. Previously, many people harvested Tyrian purple dye from very specific shellfish species—the Murex brandaris and Purpura haemastoma—gaining only a small amount of the pigment from each creature. As a result, purple became elusive and treasured, eventually gaining association with various royal families, even to the present day. Until her passing, Queen Elizabeth II donned purple during her public appearances quite frequently, and memorabilia created in her honor after her Platinum Jubilee and passing were almost exclusively designed in purple. However, violet coloration still reigns supreme as the rarest color to be seen in nature, expressed only by very specific nanostructures, whereas purple is expressed through pigments. 

The rarity of purple truly paints the diverse but also incredibly in-depth processes of our natural world. Purple demonstrates that it is more than just a vibrant, shimmering hue. It is a beautiful example of what else we can discover through science. Nowadays, most people consider purple as nothing more than just another color, but it truly once was the color of kings.