The commonest form of color wheel shows the three subtractive primaries, Red, Yellow, and Blue, with equal spacing.
The Munsell system of colors, which I think is one of the best color systems, uses a circle with ten hues: the five colors Red, Yellow, Green, Blue, and Purple are equally spaced.
And I remember a news item where Red, Yellow, Green, and Blue were identified as the "psychological primaries".
And of course color television and your computer monitor uses Red, Green and Blue as additive primaries, and color printing, unlike color painting, terms the subtractive primaries Magenta, Yellow, and Cyan.
Well, here's my little addition to the general confusion. The color circle below is based on my notion of how to subjectively space the colors generally acknowledged as primaries:
Red is closest to Yellow. Blue is further away from Red, since those two colors stand at opposite ends of the spectrum. And Blue is farthest from Yellow, since Green is percieved as almost a primary: and the distance between Blue and Green equals the distance between Green and Yellow, and both are less than the distance between Red and Yellow.
A nice color circle meeting those conditions with 60 divisions can be produced by using the sequence of numbers 4-5-6.
Red to Orange and Orange to Yellow are each 8 hues apart.
Blue to Purple and Purple to Red are each 10 hues apart.
Yellow to Green and Green to Blue are each 12 hues apart.
And here's the color circle I first arrived at on that basis:
I've modified the scheme considerably since starting these pages; here is the revised color circle, with 240 hues in the color circle:
it acknowledges that the green phosphor in a CRT should be considered as representing quite a yellowish green, for example, compared to my initial attempt.
Now, before continuing, I should address one objection that some people have to color circles like the one above.
Pure light with a wavelength of 650 nm is red; pure light with a wavelength of 575 nm is yellow; pure light with a wavelength of 475 nm is blue.
There is no wavelength of light which, by itself, looks purple. Yet purple is on the fundamental circle of saturated hues used by artists. Are they wrong: is purple only a "mixed" color, like brown or beige?
To answer this question, we need to know something about how human color vision works. The retina has two kinds of receptors, rods and cones. The rods are primarily sensitive to blue and green light, and are used for night vision. For normal vision, the cones serve, and there are three kinds of cones. Each kind is sensitive to a range of colors, but each one has a different peak of sensitivity.
We determine the color of objects we look at, or light we see, by means of the ratio between the stimuli experienced by the three kinds of cones. That isn't the whole story, because the brain performs sophisticated processing to correct for variations in the overall color of a scene due to changes in ambient light, for example, from noon to sunset, or from outdoors to indoors, under either incandescent or fluorescent light, so that we can consistently recognize objects by their own colors. But it is the starting point for color vision.
If one uses a triangular chart like this:
to plot the relative intensities of the stimuli to the three types of receptors for the different frequencies of light, one will get a curve going around the point representing the proportions of stimuli caused by white light.
If one considers that hue refers to the direction from white in which a color lies, one will note that since:
that new hues, that cannot be produced by any combination of a pure spectral light with white light, can be produced by combining red light and blue light.
The CIE chromaticity diagram, although it is based on tristimulus values obtained by experiments involving subjects comparing colors, and not directly on the spectral sensitivities of the three pigments found in the cones of the retina (which are now known) illustrates this principle, as it consists of an arching curve around white, with a straight line between red and blue accounting for the hues normally thought of as purplish.
Thus, if one wishes to be able to describe colors, on the basis of three co-ordinates; one being brightness (luminance), one being how colorful or gray a color is (saturation), and a third being which saturated color a color tends towards (hue), one cannot describe all colors unless one's circle of hues is complete, and if it only includes spectral colors and is missing purple, then one has cut a wedge-shaped chunk out of the CIE chromaticity diagram - which is considered to be scientific.
So purple really is one of the fully saturated colors. Don't let anyone tell you otherwise.
Relating the mixtures of red, green, and blue phosphors on a CRT to the colors of pure spectral light is difficult, but based on information I have found, this chart may have some approximate validity:
In addition to a color circle of fully saturated hues, there are also, of course, less saturated colors.
The following diagram includes a full gamut of such colors, with only 30 hues in use. To make it easier to calculate the RGB values for the palette I needed, and to make it easier to draw the diagram in which the colors were placed, I did not attempt to make vertical planes of constant brightness, but instead simply moved from the circle of fully saturated hues towards white and black, somewhat as is done in the Ostwald color system (but I use squares instead of hexagons, having 1, 3, 5, 7... colors instead of 1, 2, 3, 4... in successive shells):
I used a BASIC program to generate the color values I required, and its source code is available for those interested.
Using a modification of that program, whose source code is also available, I wrote a program that generated HTML for a table using background colors to show a color gamut, but this time with colors arranged by brightness (instead of by degree of white or black admixture, as in the chart above).
For a gamma of 1.00, this illustration shows a color chart made up of colors in that arrangement: the diagrams, in three rows, show every second hue, starting with red, for 24 different values of brightness, excluding black and white, and 9 different values of saturation, excluding no saturation. The diagrams have ten columns, however, because the gray scale is included with each diagram.
This diagram is in the PNG file format, which is not supported in some older browsers. It has been necessary to use that format for this file, as JPEG did not give acceptable results without an excessive file size, and it has more than 256 colors.
Note also that this color scheme is not the Munsell system, despite a superficial resemblance. In addition to the different arrangement of the hues, no attempt is made to correct for apparent hue shift for less saturated colors.
This page, if viewed in 256 color mode, will simply look less attractive, due to dithering. As background colors are not dithered, the page with the more extensive gamut will simply show incorrect colors if viewed in that mode.
Because on many computer systems, RGB values are not linear specifications of brightness, it is necessary to take into account gamma correction to present colors accurately. The charts on this page, except for the chart of "internet-safe" colors, which need not be adjusted, as browsers display those colors without correction, have been corrected by being raised to the exponent 0.85.
The following table should allow you to determine the gamma correction required for your computer display. The gray squares attempt to achieve the same shade of gray directly, and by alternating white and black pixels (or full-intensity red, green, and blue pixels in one case): where the vertical bars in each square are the closest match in apparent brightness, the correct gamma for your monitor is found.
The first item in each row is the chart for checking gamma. The second item is my color circle, presented correctly for systems requiring the gamma correction indicated by that row. The third item is the gamma value:
 |
 |
1.00 |
 |
 |
0.90 |
 |
 |
0.85 |
 |
 |
0.80 |
 |
 |
0.70 |
 |
 |
0.60 |
 |
 |
0.50 |
Of course, there are other ways to organize a gamut of colors that includes possible colors of all kinds to at least approximate any color. The "internet-safe" colors that work in 256 color mode on common browsers are an example of what is known as process color, and are shown here:
This can be thought of either as a chart of additive colors, or as a chart of subtractive colors. Starting from black, one goes up to add blue, to the left to add red, and away from the observer to add green. Starting from white, one goes down to subtract out blue light using yellow ink, one goes right to subtract out red light using cyan ink, and one goes towards the observer to subtract out green light using magenta ink.
However, in fact, the chart depicts colors produced by addition from monitor phosphors, and not subtraction. Also, in practice, while yellow ink indeed looks yellow, cyan is more like a light blue, and magenta more like red than like purple, than the colors in this diagram.
A similar process color chart with colors more closely approximating what might be available through printing is shown below:
By default, Microsoft Windows uses a different palette of process colors for bitmap images in 256 color mode. Instead of using only 216 colors, all 256 colors are used: the brighter components red and green vary through eight steps rather than six, while only four levels are allocated to blue. This produces a range of colors having the following appearance:
The colors chosen tend to favor the darker colors: for blue, the levels are 0, 64, 128, and 255, with 192 omitted, and for red and green, the levels are 0, 32, 64, 96, 128, 160, 192, and 255, with 224 omitted.
Favoring the lighter colors instead, and allocating four levels to blue, five levels to red, and twelve levels to green, using:
Blue: 0 153 204 255 Red: 0 153 187 221 255 Green: 0 85 102 119 136 153 170 187 204 221 238 255
one obtains the following graph:
In addition to the 240 colors formed systematically, the missing gray shades with 85, 102, 119, 136, 170, 187, 204, 221, and 238 for all colors are added. As that left seven colors unused, colors were added to cover some of the darker colors, adding all combinations of 102 for Blue, 119 for Red, and 51 for Green, on and off (except for all off, black, which was already present).
Just as the circular chart that began this section represents the color world as seen by a painter, who has a palette of oil paints representing several saturated colors and also white and black paint, the process color charts represent the color world experienced by someone working with a color monitor, or with printing.
To make a particular color, a painter should use the two saturated colors nearest in hue to the desired color, plus as much white or black paint as is needed to make a good match.
When printing a color photograph by conventional color separation techniques, not involving the use of computer-generated halftones, it is necessary to use cyan, magenta, and yellow to produce the various colors. A light black-and-white image of the same photograph is also overprinted.
But ideally, if one is trying to produce a desired color directly, one should mix at most two of cyan, magenta, and yellow inks with as much black ink as required. For almost the same reason that a painter should not try to create a desired color using widely separated hues. In the painter's case, too much trial and error is needed that way; in the printer's case, the limitations of the inks used are more important.
Color Atlas by Harald Küppers featured a set of color squares for this kind of system; it was an inexpensive small book with a silver cover. Below is a diagram of what a process color chart looks like in that kind of system.
The first row shows colors built from yellow and magenta; the succeeding squares going to the right have more and more black added. The set of nine separated squares have 75% black added, and, since that makes them hard to distinguish, only nine are shown.
The second row shows yellow and cyan, and the third row magenta and cyan.