I was rather upset, a week or so back, to hear some supposed experts talking about red, yellow, and blue as a set of primary colors on my local NPR station one morning. This, in the same paragraph in which these same people explained about the "red, green, blue" and "cyan, magenta, yellow" primary sets. Argh.
Look: first of all, there is no law that says primary colors have to come in triads. None. In fact, I have seen holograms made in "The Union Formerly Called Soviet," as a friend of mine calls it, that were created with light from a pulsed xenon laser, which means that they used five primary colors.
The truth about red, yellow, and blue (and this is not original with me: I read it in a book on color theory) is that they are 3/4 of the usual psychological primary set, which is Red, Yellow, Green, and Blue. (Why did you think those wonderful 3M sticky-flags are most commonly sold in those four colors, anyway?)
Why people teach kids in school that RYB is a full set of primaries is beyond me. That kind of crap just leads to the infamous "mud-people" -- and you can never get a really good green by mixing pure yellow and pure blue. It turns out that there is good and sufficient reason for this. (It also turns out that there are yellows and blues that do mix to more-or-less bearable greens; but that's a matter of clever choices, not of basics.)
I want to explain about RGB and CMY, because most
of the explanations I've seen are nearly impossible
to understand, and I just don't think it has to be
that tough. Sure, it's a bit tricky, but this is
decidedly not rocket science.
We typically do additive color using Red, Green, and Blue as primaries. (You can validate this claim by sneezing while sitting in front of your monitor. Ahem.)
Please note that I said, "typically". You don't have to do it this way if you don't want to. I probably won't get into detail on that here, because I want to explain the way it is typically done, and that's tricky enough.
So. This is called "additive" color, because we are going to start with black. (You can validate this claim by taking your monitor into a pitch-dark room and turning it off. Ahem.)
If we just turn on a pure red light, what we see is (no surprise) red. Pure green, sure enough, looks green, and blue gives us blue. If we turn on red and green together, however, in roughly equal amounts, we get a new color. It is called yellow. Now, it is very important to note that in this system (an additive system using RGB primaries), yellow is a secondary color -- that is, a color you make by adding other colors together. (When we do this on your screen with numbers, later, we'll find out something else about this kind of mixing.)
If we turn on green and blue together, we get a new color, which is called cyan. Similarly, if we turn on red and blue together, we get a new color, which is called magenta. If we turn all three of them on, we get white.
As you would expect, if you take a whole lot of red and you add only a little green, the red shifts a little bit into the orange. If you take a lot of green and add only a little bit of red, you start getting into chartreuse.
Similarly, if you add modest amounts of red to a full-on blue, you get purples, and so on. (You can see examples of all of these and more on the Web if you follow the links in the section on "Doing it by the numbers." In the Hex Triplet chart of the first link, "FF3300" is already a fairly orangy red color, and "CC00FF" is rather purple, while "3300FF", which has a lot less red, is still rather blue. ...And so on.)
Also, of course, if you add roughly equal amounts of all three, you get white. Well, if you add roughly equal small amounts you get gray, but it's the same idea.
If your computer can display "24-bit" color (also referred to as "true color"), you can see samples of lots of these colors on the Web. One set is at this page; another starts at this page, on William R. Van Kuyk's Web site, and continues for three more pages. These are important here because they list some numerical equivalents of the various colors, to make it easier for you to use them on your own Web pages. Those numbers, however, are specific to computers. You can generate lots of colors just as easily with slide projectors, too, or with LEDs; and if you do so, the numbers don't apply.
In terms of hex number (I'll explain this later, in case you don't already know about it), as you can see in those Web pages, each primary color is represented by two digits, with 00 standing for "none", and FF standing for "100%". The first two digits are for red, the middle two are for green, and the last two are for blue.
Thus, full-on red is FF0000, green is 00FF00, and blue is 0000FF. Full-on red plus full-on green, FFFF00, is yellow, cyan, which is green plus blue, is 00FFFF, and magenta is FF00FF. White, of course, is FFFFFF, with all three colors full on. The background of this page, a rather light gray, is F4F4F4.
This brings up an interesting and important point: If
"FFFFFF" is white, and if we take away blue, leaving
"FFFF00", we get yellow. We're going to return to this
idea in a little while, because it's important.
In case you are not familiar with "Hex" (it stands for
"hexadecimal", and means "base-16") notation as it is
written in the computer world, "00" is zero, just the
way it is in regular numbers, and "FF" is equivalent to
255. Because there are 256 possible amounts of red
(counting "none" as an amount), 256 possible amounts of
green, and 256 possible amounts of blue on a good display,
we can make a total of 256X256X256 colors. That's a little
over 16 million. Oddly enough, even though your eye can
probably distinguish only about 7 million colors, you
can still see colors that can't be made on screen. I
will try to say more about that at the end, if I have
time and energy.
As I already said, if we want white we add our three primaries in balanced amounts. If the balance is off, the "white" is tinted. In fact, even if the balance is correct, the white is, technically speaking, tinted. It has a characteristic "color temperature", which has to do with physics and is a bit beyond the scope of this page. (It's important, though, especially if you are doing printing, TV, or color photography.)
Remember how we could get yellow by starting with white and taking away blue? We can just as easily make all of our colors that way. This is, in fact, how color printing works. You start with a surface that is approximately white, and you print inks on it to take away some of the colors to leave the color you want. Traditionally, this is done with colors that amount to "minus blue" (yellow), "minus red" (cyan) and "minus green" (magenta). (If you don't like "minus", try "subtract", "negative", "inverse", or "take away the".)
The inks have to be transparent, so that you can mix them. If they weren't, you'd only get the color that landed on top. Also, because inks are considerably less than perfectly matched, it's not possible to get a really crisp black by printing all three colors of ink, so most color printing is done with the addition of black as a "fourth color". This color set is called "CMYK", where "K" stands for black. (We can't use "B", because that means blue.)
Color negative film and color prints are a little harder to understand because of the way photography works. I am not going to attempt to explain them here unless someone asks really nicely. (Color film is rocket science.) Most common color prints, though, almost certainly use a CMY set of colored dyes, which are matched very carefully so they do produce a fairly crisp black when all three of them are present.
We could just as easily say that cyan, magenta, and yellow were the additive primaries, and red, green, and blue were the subtractive ones, but we generally don't. We could also use other sets, but we don't usually do that, either. Sometimes it is nice to have a standard, and to stick to it. (Sometimes it is nice to extend it: some high-quality printing is done with CMYK and Brown inks.)
First off, I should note that this way of doing colors is not by any means the only way. If you are interested in color, you should look up Göthe's color theory, and the work of Edwin Land as well. I don't have time to say anything substantive about either of those here, but they are quite important.
Second, it is clear that human DNA varies somewhat. I read, a long time ago, that there were at least three different possible green receptor pigments in the human eye, and more recently I was told that there are lots of different red ones. (Yes, the human eye appears to use red, green, and blue as primary colors. Yes, it's more complicated than that. If you want to try to understand human color vision you should start reading as soon as possible, and don't believe everything you read without checking at least some of it.)
In terms of "pitch", by the way, your eye can discern just about one single octave, from 400 nm (deep violet) to roughly 800 nm (far red). The actual values at both ends of the spectrum vary from person to person. I can easily see out to about 820 nm, for example, and some people can see even further, but many textbooks claim that anything beyond 700 nm is invisible infrared! Also, your night vision covers a slightly different spectrum from that of your day vision, and has other interesting characteristics as well. As I said, start reading now -- you've got many pages ahead of you!
Now, there are some odd things that happen if you make colors by playing "mix-and-match", especially if you happen to use lasers. Most lasers produce relatively pure colors. If we use a cadmium laser for blue (at 441.6 nm), a thallium laser for green (at 535.0 nm, thallium is about the greenest green that my own eyes can perceive), and a diode laser for red (at 650 nm), we can get a fairly good white. Now, what happens if you take this set of lasers into an otherwise dark room, and shine them all on an orange object that just happens to reflect extremely well from about 560 nm to 630 nm, and not very well at all at other wavelengths? Surprise, it looks almost black. Take it back out into the clear light of day and it's bright orange again.
If you take a piece of paper and make a small hole in it, and you look through that hole at the skin of an orange, it looks, well, orange. If you look through the hole at a piece of milk chocolate, it looks... well, orange. Hmmm. Most browns turn out to be red, orange, or yellow. (Take an empty brown glass bottle, put it in a beam of sunlight, and examine the shadow it casts. Looks pretty yellow to me!)
It was discovered a few years ago (I think I saw this mentioned in Science News, a wonderful little weekly magazine) that people can sometimes perceive colors that can't actually exist, like "reddish green". If I recall correctly, this happens when small areas of contrasting colors (red and green, yellow and blue, etc.) are printed close together, but please don't trust my memory.
Before I close, let me mention one book:
Color Perception in Art
Faber Birren, 1986
Schiffer Publishing, Ltd.,
77 Lower Valley Road
Atglen PA 19310
ISBN: 0-88740-064-7
Birren has several books out, of which this is the only one I have seen. It gets into considerably more detail than I do here, specifically orienting itself toward art rather than, say, computer monitors or printing. Also, it is more about perception and the subtleties thereof than anything as simple as color triads.
There's a whole lot more, but it doesn't come to mind
right now, and it's past my bedtime anyway. Good night.