Earlier this month, on a cloudy winter morning, I set out to take photographs along the southern shore of Lake Erie. It was a dark day, the kind of day when everything looks gray. It is not that the colors had disappeared completely, but nearly so; there was not the usual sharp contrast between red brick and the blue of the lake.
When you stop to think about it, though, the question arises: how is that possible? How can a very dark cloudy day turns everything gray?
Human eyes lose some of their ability to see colors in dim light, but that is not the answer here. The darkest, cloudiest day is still ten times as bright as a video display, such as the computer monitor you are likely looking at right now as you read this post, and we see colors on a computer without any difficulty. A dark gray day is also much brighter than the fading reds and purples of sunset, and those are some of the most vivid colors we may see on a daily basis.
At the same time, the grayness of a gray day is not just an optical illusion or an accident of human vision. The camera I had along on my trip recorded some of the same grayness that I saw with my eyes. You can see this yourself if you look at the photographs I took. So it is not that a dark gray day is so dark that we are no longer seeing colors. Rather, the color of the light is turning things gray. So what color is it that turns things gray?
It is not gray light. If you tried to create gray light, it would just be a less bright version of white light.
The answer, as it turns out, is green. Green light turns things gray. To understand why this is, you have to know what green is — and first, you have to know what visible light is.
Visible light is the light that the human eye can easily see. It is electromagnetic radiation within a relatively narrow range of wavelengths that the eye is sensitive to. And this range of visible light is not an arbitrary limitation or accidental quality of the human eye. Visible light covers the entire range of sunlight that can get through Earth’s atmosphere efficiently.
As the short end, the ultraviolet light that the sun emits just as energetically as visible light is blocked by the ozone layer. The ozone layer, a thin layer of the upper atmosphere, absorbs at least 97 percent of ultraviolet light. On Earth, only 1 to 3 percent of the sun’s ultraviolet light makes it through to the surface. At the long end, infrared light does not travel long distances well. It is absorbed, along the way, by any gas, dust, water, or other material that might be present.
Visible light, then, is essentially sunlight at ground level. It is the range of light wavelengths that can cut through the atmosphere all the way to the ground.
The middle of this range is green. The green band is right at the center of our ability to see. It is the color humans, and mammals generally, are most able to see. We may respond about equally to red, green, and blue, but we have a much greater ability to see green than to see red or blue. Green is the base color of human vision. All other colors — including white and black — are understood by comparison to green.
Green is also everywhere in our natural environment. Almost every natural color we see has a significant amount of green in it.
The exceptions are the colors that are specifically meant to stand out. The reds and purples of flowers are the anti-greens, clearly distinct colors that insects can find easily as they pollinate flowers. Among artificial colors, the red of a stop sign or red light is meant to stand out and grab our attention. The violet and ultraviolet of a black light gets our attention in a different way. Other than colors like these, though, almost all colors are based on green.
But if red and purple are the anti-greens, green is, in a way, the anti-color. There is so much green everywhere, we easily tune it out. That is how green turns into gray.
A gray day is actually a green day. (I don’t have any indication that this is the meaning indicated by the name of the rock band Green Day, but I am happy to have an excuse to mention them here regardless.) If the clouds or fog are thick enough, only the middle of the visible light band can squeeze through easily. The middle, as I mentioned, is the green band. If most of the light that gets through to where you are is green, you don’t say that the world has turned green. Green is the base color, so it is the color we tune out most easily. If everything is green, then everything is gray.
This helps to explain why you so rarely see green stage lights. If you put something on a stage, you want it to look colorful. But shining green light on it would make it look dull and gray — the opposite of the effect you want. Stage lights, then, are traditionally white, red, yellow, and blue. Some modern lighting designers add cyan for completeness. You almost never see green in a lighting array. There is no need for it. You get a better effect, at a lower cost, by dimming the white lights.
All mammals, as far as we know, share the same sensitivity to green that humans have. It helps to explain why all grazing animals are mammals. Mammals, perhaps, have a special ability to find grass because of this special sensitivity to green.
Grass, graze, and green are, I believe, all variants of the same ancient word. It may be more of a coincidence that gray and ground begin with the same sounds, but if so, it is a meaningful coincidence. Green and gray are identified with each other at times, as I have explained. Green is the base color, but more than that, it is specifically the color of the ground. And that explains the need for blue and red, which is the subject I will turn to next.
Previously: Three Colors.