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Colors are perceived by the cones in the retina of the eye. Below shows the effectiveness of the three types of cones as a function of wavelength.
The cones for convenience are named red, green and blue. As will be shown, blue is a misnomer.
The efficiency curves for the red and green cones cross (i.e., are exactly equal) for radiation of wavelength about 0.56 μm. When the eye sees 0.56 μm radiation the red and green cones are stimulated about equally. The visual perception of near equal stimulation of the red and green cones is yellowness. The radiation of wavelength 0.56 μm itself does not have a color anymore than microwave radiation or radio waves have a color. Yellowness comes from equal stimulation of the red and green cones. Light including radiation of equal intensity at the 0.58 μm and 0.54 μm wavelength, the wavelengths of maximum efficiency for the red and green cones, would also be perceived as yellow light. There would be some stimulation of the blue cones by the 0.54 μm radiation which would lighten the yellowness of the perception.
The efficiency curves for the cone receptors are shown in the above diagram as going to zero, but they likely taper off asymptotically to zero like a Gaussian curve. The reason for saying this is that very high intensity light from a laser emitting infrared light is perceived as being deep ruby red. The infrared does not have a color but its intensity is so great that it stimulates the red cones in the tail of the efficiency curve where the efficiency of the perception is small but nonzero. The redness perceived as a result of the infrared laser is the output of the red cones unmixed with any stimulation of the green and blue cones. The redness of this perceived color indicates that it is appropriate to called the cones responsive to relatively long wavelength light the red cones.
It is a different story at the other end of the spectrum. When white light is passed through a glass prism the light is separated into its components. It is the spectrum of the rainbow. Each minute segment of the rainbow represents light of a particular wavelength. The shortest wavelength light stimulates only what are called the blue cones. But the end of the rainbow is not blue; it is violet. This means that the output of the so-called blue cones is violet color. Such cones should be called violet cones.
Blue color results from the stimulation of the green cones along with the violet cones. As seen in the diagram light of wavelength 0.44 μm stimulates the violet cones (the so-called blue cones) to a maximum degree but it also stimulates the green cones to about 20 percent of their maximum. The red cones would be stimulated to an infinitesimal degree as well but this can be ignored. So the perception of blueness comes from a stimulation of the violet cones 100% and the green cones 20%. The red cones would be stimulated to an infinitesimal degree as well but this can be ignored.
Untold generations of students have learned that violet is a color created by mixing red and blue. Violet is perceived to be a mixed color. Blue, on the other hand, it considered to be a primary color.
Now the problem is explaining how the combination of red and blue light produces violet. To do this it is convenient to do so in terms of the Red-Green-Blue color model.
In computer graphics colors are giving by their RGB (red-green-blue) values, where the values run run from 0 to 255. Suppose a color has the RGB values of (255, 100, 100). The levels of 100 for both the Green and Blue values combine with 100 of the red value to create a dark gray (100,100,100).
The left-over value is (155,0,0) which is a dark red.
The combination of this red with the gray value is a lighter red.
It is important to remember that the color values add rather than average. The darkest gray will lighten another color at least a little.
Equal values for red and green produce yellow. A color whose RGB values are (255,255,100) can be consider to be the sum of a dark gray (100,100,100) with a yellow of (155,155,0).
The combination is a lightened yellow.
In the RGB color model the brightest violet is (255,0,255).
The creation of a violet from red and blue appears to be a puzzle since violet involves a shorter wavelength of light than blue. The answer is that the proper color model based upon the cones of the eye is an RGV (Red-Green-Violet) model. In an RGV model blue would have RGV values of something like (0,150,255). When this is combined with red light of say (150,0,0) the result is (150,150,255) which is like a lighter gray (150,150,150) combined with a darker violet of (0,0,105) which is a lightened violet.
A combination of a pure red (255,0,0) and a blue (0,150,255) would be equivalent to a gray (150,150,150) combined with a (105,0,105) which is a reddish violet rather than a pure violet. The combination is thus a lightened reddish violet.
Producing a pure violet in the RGB color model requires ascertaining the level of green involved in a pure blue. If red of the same level as the green in a blue are combined the result is a lightened pure violet.
Any color of course can be consider a primary color, but the primary colors that correspond to the cones of the retina of the eye are Red, Green and Violet.
If violet is primary then a combination of violet and green should produce blue. Here is an example:
If one observes a spectrum created by a prism it is clear that the bands for red, green and violet are significantly wider than those for yellow and blue, approximately two to three times as wide. This is entirely an effect of the the functioning of the cones of the eye and not related to the physical distribution of wavelengths of light. It is clear that the efficiency diagram should be properly labeled as shown below.
The color circles for red-green-violet primaries is shown below:
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