Green, along with red and blue, are the three main colors of human vision. Green, as I explained, is not an arbitrary color that humans just happen to be able to see, but is the most important color on Earth. It is the center of the wavelength band of ground-level sunlight, and the core of the human ability to see. Green is the color we see better than any other.
If green is not a random or accidental color, then red and blue are not accidents either. Blue, actually, comes next. If green is the color of the ground, and especially of plants on the ground which may be almost pure green, it helps to be able to see a second color that does not belong to the ground. Having a second color makes it easier to distinguish ground from non-ground. The color of non-ground is blue.
In physical terms, blue belongs to wavelengths that are near the short end of the visible light band. If you can see blue and green, then you can tell a short wavelength from a middling wavelength, and you can tell a fluid from a reflecting surface. Fluids tend to be transparent, but whenever they are thick enough or deep enough, they are blue. Water is blue. The sky is blue.
Fluids are blue for the same reason that infrared is not part of the visible light band. Longer wavelengths, especially infrared but also red and green, are more quickly absorbed as they travel through air or water. Shorter wavelengths, the wavelengths of blue, get farther, so they are the ones you see the most of.
In physical terms, the sky may be just as green as it is blue, but green is the base color, while blue is a contrasting or dissenting color, so it is the blue we notice. It is not so important to have an enhanced ability to see the sky itself — whether it is blue on a sunny day or black at night, it is the one feature of our physical environment we can’t miss. But when the sky is reflected in water, the blue of the sky makes the water that much more visible. And water is a subject of importance. We won’t survive for much more than a day without it.
The existence of blue also makes it possible to separate gray from green. When a color is strong in green, it could be either gray or green. It is green if it is not so strong in blue wavelengths. If it is equally strong in green and blue, then chances are, it is some kind of metal or stone (a metal oxide), and this comes across as gray or another neutral color.
But if the ability to pick out colors that fall toward the short end of the visible light band is useful, it may also be useful to pick out colors that fall toward the long end of the visible light band. That, as you may have guessed by now, is where red comes in. Red, like blue, is a dissenting or contrasting color, but in the opposite direction, the direction of the longer wavelengths.
If blue is the color that leads us to water, red is the color of food. It is true enough that some of the most important food is green. Some of the most interesting and energy-dense fruits and vegetables, though, are red, or they are orange and yellow, which are colors that mix red and green, but lean toward red.
This association of red with energy goes beyond food. Energy in its more concentrated and active forms tends to be more red. Humans, for example, may come in various colors and shades, but all of our skin colors are more red than green or blue. Fire may be seen as red and yellow. Anything that gets warm enough to glow gives off infrared first, and then, if it is hot enough, red. Blue, by contrast, is more often the color of a passive approach to energy, merely absorbing it, but not organizing it.
Having red in addition to blue and green makes it possible to sort out the neutral colors. They are not all gray. They may be tan, brown, beige, and so on, depending on the strength at the red end.
We can confirm the sequence of colors in human vision, with red last in the list, by looking at genetic statistics. Deficiencies in sensitivity to blue are rare, but difficulties with red when compared to green are startlingly common. Among people with healthy eyesight, only about 92 percent of males and 99.6 percent of females worldwide have a full ability to distinguish red, yellow, and green when viewing printed documents and color-coding under artificial light. The distinction between red and green is useful, but not as fundamental as the distinction between blue and green.
If green is the base color of human vision, the colors with the least green are the “look at me” colors that stand out the best. These are the deeper blues, violet, red, and black, and any color in between. Plants take on these colors for flowers and fruits when it is especially important to get the attention of animals.
Black has become so commonplace in everyday life that it is hard to think of black as an accent color, yet it is black’s tremendous visibility that makes people choose it for so many things. You are reading this text, most likely, in black (or very dark gray) on a white background. It is traditional to show text in black on white, and for good reason. Long before there were printing presses, scribes learned that black and white was the combination of colors that lent the greatest clarity and importance to the written word. Black clothing, in a similar way, is common not just because black is the easiest color to apply to fabric, but also because black is one of the boldest and most conspicuous colors for a person to wear — or it would be conspicuous, that is, if there were not so many other people also dressed in black. Black is not nearly so common in the natural world, and where it does occur, in a black flower or the sand of a black sand beach, it is hard to ignore.