I got into a discussion with an art teacher, and we’re kind of stuck at the point about brown being clearly visible to humans, but not being in the rainbow. I know why black and white aren’t colors, but what do we say for brown? Apparently the rainbow does not cover all of the possible perceptions by humans.
One simple way to see this: A rainbow contains a one-dimensional continuum of colors. That is to say, given a rainbow, you can specify a color in the rainbow with a single number, to say where on the rainbow it is. But the full space of colors visible to humans is three-dimensional. You need three numbers to specify a general color, such as the R, G, and B values used in a computer monitor.
Actually, RGB numbers don’t completely specify all of the colors humans can see, either: There are some colors that, to reproduce them exactly, you’d need a negative value on one or two of those numbers. But RGB-space is at least a pretty good approximation of the full color space.
One other point that bears stressing: Although you often see discussion of colors in introductory physics courses, it’s really not a physics topic. It’s all about what humans, and humans specifically, can perceive. Some animals can distinguish between colors that would look completely identical to humans, and a sufficiently-advanced sensor can distinguish even more than any animal.
You mean dark orange?
It’s not typical terminology to say that white, black and brown are not colors. They are indeed colors, but they are non-spectral colors.
Some references:
Yes - one way of looking at brown (at least in subtractive color mixing theory, that is, mixing colors from paints rather than from colored lights) is as a shade of orange. By that I mean: take orange paint, add black, and you get brown.
I am basing this on the simple color theory system I learned in middle school, whereby:
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a “shade” is defined, not as a nuance of a color (i.e. indigo vs. cobalt vs. royal blue), but as the color with an admixture of black (similar to a shadow)
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a “tint” is defined as a color plus white (thus pink is a tint of red, similar to being exposed to very bright light)
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a “tone” is defined as a color plus gray (or alternatively as a color plus black plus white)
Perceived color is not determined only by the spectrum of the light. It is also dependent on the contrast with other things in the field of view. Brown is a darker version of yellowish orange. You can easily find examples on the internet. For example a cube with a brown square on the brightly illuminated top, but the same color is orange on the less illuminated side.
I’ve written a column for Optics and Photonics News about brown. I can send you a copy, if you’re interested.
Brown is, in essence, dark yellow or dark orange. You don’t see it in a rainbow because you don’t get the full range of brightness. In a dark enough rainbow, brown would be present.
As noted above, purple and its variations (magenta, pink) are not part of the color spectrum, and are the result of mixing colors from the blue and red ends of the spectrum. But, to be fair, it’s not correct to say that you don’t see them in a rainbow. Pink and aqua are the colors of the Supernumerary Rainbow, which you can see if all the droplets are of fairly uniform size. In that case, you have colors mixing inside the blue edge of the basic rainbow. You also see pink and aqua when you have multiple-order white light interference, as in soap bubbles and oil films.
Depends on which RGB space you’re specifying. The ACES CG Linear color space incorporates all human perceivable colors plus a bunch of imaginary colors out in the corners.