The natural world is filled with dazzling color. From lush green fields and sunny yellow flowers to red birds and iridescent blue insects, you can see something truly incredible each time you step outside.
But as you’ve likely noticed, some colors are more common in nature than others. Green is easily one of the most abundant, as is brown. But blue (if you remove the sky and blue water from the equation) is anything but.
Most vision and color experts believe that blue is nature’s rarest color. Our eyes can also be deceiving — in many cases, what we see as “blue” just comes from patterns of scattering light.
So what makes blue so rare? And how do we know if something we see is truly blue or just an optical illusion? Let’s take a closer look.
What Makes Us See Blue (or Any Color)?
When we see pure light, it looks white. But white light is actually made of every visible color.
Of course, we can’t see every form of light. For instance, infrared light is made up of longer wavelengths than we can see. Ultraviolet light has shorter wavelengths than we can see. So the spectrum of visible light sits right between infrared and ultraviolet.
Here’s a cool illustration of visible light and other electromagnetic waves:
When visible light hits a given object, that object absorbs most wavelengths of visible light. However, it reflects some of the wavelengths, and those are the colors we see.
For example, let’s say you have a blue quilt on your bed. When the sun shines through your window and onto the quilt, the quilt absorbs all the wavelengths except for the blue ones. The blue wavelengths of light are reflected toward your eye.
As light moves through your eye, it reaches specialized cells called “cones.” These cells are located in the retina, and they respond to specific wavelengths of light. Only about 2% of the cones are sensitive to blue light. For the sake of comparison, about 64% of cones are sensitive to red, and 32% are sensitive to green. Once the light from the quilt reaches the cones in your eyes, you perceive the quilt as being the color blue.
Why Is Blue So Rare?
Most colors on the spectrum of light are fairly common in nature. So what’s so special about blue? For a plant or animal to appear blue, it must be able to absorb other wavelengths of light.
The closer you get to infrared light, the lower the energy of the light. As you get toward ultraviolet light, the light has much higher energy.
According to Kai Kupferschmidt, author of a book called Blue: In Search of Nature’s Rarest Color, that means that for an object to appear blue, it must be able to produce a molecule that only absorbs small amounts of energy. That’s harder to do than it sounds! These molecules tend to be large, very complex, and very difficult to produce, so they aren’t too terribly common.
It makes sense — when you think about it, blue is rare in the animal, plant, and mineral worlds. Let’s take a closer look at how blue appears in each.
Blue in the Animal Kingdom
Quick — think of a blue animal! There aren’t too many in the natural world. Maybe you pictured a bluebird or a blue morpho butterfly.
But did you know that many “blue” animals only appear blue as the result of an optical illusion? Researchers believe that only about 1% of animals are truly blue. The obrina olivewing butterfly and the blue poison dart frog (shown above) are among the very few animals that actually produce blue pigment.
So if most other “blue” animals aren’t really blue, what makes them appear that way? After all, this puffy bluebird looks unmistakably blue:
Scott Sillett, a biologist at the Smithsonian Migratory Bird Center, says that there are no birds that are truly blue. In birds (as well as some butterflies and other animals), blue is what’s known as a “structural color.” That means that the arrangement of keratin molecules in the bird’s feathers reflects light in a certain way. The wavelengths of reflected light appear blue to our eyes.
But what happens if that structure breaks down? If you were to find a blue jay’s feather on the ground and (for some reason) decided to grind it up, it would no longer appear blue. If you destroy the complex structure of the feather, it will no longer reflect only blue wavelengths. Without that structure, the feather would likely look brownish or grayish.
Notable, most other colors in birds (and many other animals) are not structural. When you see birds of different colors (like red or yellow), their feathers are colored by actual red or yellow pigments. If you were to grind up a red or yellow feather, the resulting dust would still be red or yellow.
Blue in the Plant World
In the plant world, blue is primarily produced by a class of pigments called anthocyanins. However, anthocyanins are not always (or even often) blue. They are highly unstable molecules, and depending on the acidity of their surroundings, they can appear red, purple, blue, or even black. Their coloration can also vary based on sugar molecules or other molecules that attach to them.
One well-known example of the instability of anthocyanins is the hydrangea plant. You already know that the color of hydrangea flowers can change based on soil pH. In acidic soil, the flowers will look more blue. If the soil’s pH is closer to neutral, the flowers will be more pink.
This color-changing process starts with aluminum. In acidic soil, aluminum becomes more soluble and is thus more easily absorbed by plants. When plants absorb more aluminum, their anthocyanin molecules move closer together, making them look more blue. In more alkaline soils, aluminum is less soluble, so the anthocyanin molecules move further apart. This makes the blooms look pink.
As you can see, several conditions need to come together to make an anthocyanin-rich plant appear blue. Consequently, truly blue plants are incredibly rare! There are an estimated 280,000 species of flowering plants in the world, and it’s estimated that less than 10% of them produce blue flowers, according to David Lee, author of Nature’s Palette: The Science of Plant Color.
Scientists had to try for years before they could create an actually blue bloom, too. In 2017, a Japanese research team did it. The process was a complex one: it began with adding a gene from the Canterbury bell (a bluish flower) to a chrysanthemum. The gene interacted with anthocyanins in the flower to make a purplish color.
The next step was adding another gene. This one came from a species of butterfly pea with blue flowers. After that, the blue chrysanthemum was born.
However, after later analysis, the researchers discovered that the flowers contained an existing compound that interacted with its anthocyanins as well. That means that the chrysanthemum’s anthocyanins needed three separate modifications in order to make blue flowers!
So why have any plants evolved to be blue? Scientists believe that blue flowers might be especially attractive to bees and other pollinators. Over time, these blue-flowered plants may have gained an evolutionary advantage over other plants.
Blue in the Mineral World
If you’ve ever laid eyes on a bright piece of lapis lazuli, you understand why cultures throughout history have chosen to use it to make blue paints and pigments. Lapis lazuli is one of the rare blue stones and minerals found in the natural world. It contains ultramarine, an exceedingly rare natural blue pigment.
Ultramarine’s blue color comes from the presence of trisulfide ions. These are groups of three connected sulfur atoms that alternate between binding to and releasing an electron. The structure of the trisulfide ions makes ultramarine (and lapis lazuli) look blue.
Not all blue minerals are colored by trisulfide ions. However, their blue coloring often comes from the presence of copper or sulfur in some form.
Blue in the Human World vs the Natural World
When you think about the color blue, you might notice a dichotomy — this pretty shade is rare in nature, but it’s very common in the human world. In worldwide surveys of the most popular colors, blue almost always comes out on top.
Maybe we’re drawn to blue because of its rarity in nature. Our favorite colors are also shaped in part by positive experiences. We might associate blue with a beach day or a fun day outside under the blue sky, and that might make us love it, too.
Either way, for most of us, seeing shades of blue in the natural world is pleasant and memorable. Next time you go outside, see how much blue you can find!