Artists and photographers often speak of certain colors which are considered to be special, being called "primary colors." By now, however, it should be obvious that these colors are not special because of any unique properties in the physics of light, but rather because they roughly correspond to the natural sensitivities of our visual receptors.

When we see the color red, it is because of the way our visual receptors have responded to the light hitting our eyes: In this case, red corresponds to a 90% response by visual pigment A. Similarly, yellow is seen when there is a nearly equal response by both Pigment A and Pigment B (with a little Pigment C); and Blue is seen when the response is distributed nearly equally amongst all three pigments. Violet, on the other hand, is almost entirely due to a Pigment C response, but it is not as common in nature as the other ones, and is therefore not considered a primary.

It turns out that our perception of color is also dependent on the intensity of light. Greater intensity light causes the perception of orange and yellow-green to move towards yellow, while violet and blue-green appear to become more blue as the intensity increases. Only red, yellow, green and blue are perceived as being independent of intensity. These colors are called "psychological primaries" because each can be said to contain no perceived element of the others, regardless of intensity. This phenomenon is not well understood but is probably related to a combination of the absorption curves of the pigments, the complex "wiring" of the retina, and the fact that light of different colors focus at different distances, with blue having the shortest focal length.

There are two other types of primaries to think about: Transmitted light and reflected light. Transmitted light primaries are usually considered to be blue, green and magenta. In 1855, Maxwell found that almost any color could be produced by mixing lights of only three "primary" colors: orange-red, green and blue-violet. The color resulting from a particular mixture can be represented by a point on a triangular grid, with each of the "color primaries" at a corner. This is the start of the so-called "color wheel" which is so important in modern art classes.

Maxwell thought that all colors might be specified this way, but in fact, this is not true. Regardless of which colors one chooses as primaries there will always be some colors which can *not* be made by mixing. The idea of color primaries is very useful, so instead of discarding the idea, people have chosen to imagine *ideal* primaries, within a graph of which, any real color might be plotted, including the real primaries.

So, when people speak of color wheels and primary colors, they are ususally speaking about imaginary and ideal colors. This is also why art classes so often compel students to mix their own color wheels: No set of pigments are ideal, therefore no two sets of *real* colors can produce identical colors!

Television and photography works on the basis of transmitted light, and hence uses the primaries listed above. Again, these primaries are somewhat arbitrary, but have some basis in how our visual system operates. Photography is understood in terms of transmitted light, rather than reflected, because light of various colors is filtered out by the various emulsions on the film, thereby allowing the *transmitted* light to be reflected back to the eye by means of the white backing card.

Artists and printers, however, are concerned with reflected light. Indeed, most colored objects reflect color. When a spectrum of light strikes an object, it absorbs most of the colors. What is not absorbed is usually reflected into the eye and perceived as a specific color, such as green. This reflected light may be either a simple reflection of incident (green) light, or it may be the result of combinations of wavelengths which are reflected in proportions which are perceived as "green", or these wavelengths may be interacting via either constructive or destructive interference.

When one speaks of reflected light color primaries, there have been a great many different sets of colors which which have been proposed. Da Vinci adopted the pragmatic approach of using six primary colors consisting of white, black, red, yellow, green and blue. Le Blon (ca. 1731) discovered the (so-called) primary nature of red, yellow and blue in pigment mixtures. The poet Johann Wolfgang von Goethe believed only yellow and blue were primary. The important color theorist, M.E. Chevreul, who essentially created the spark for Impressionism, also subscribed to the red, yellow, blue primaries. The German scientist Helmholtz pressed for red, green and blue (actually blue-violet), as did the British scientist Maxwell and American painter and teacher, Michel Jacobs. Ewald Hering, a German physiologist, developed a color wheel based upon the "psychological primaries" of red, yellow, green and blue.

In summary: All systems of color primaries assume that all colors may represented by a "circle" which starts at red, and continues through orange, yellow, green, blue and violet, and then goes back to red. This is obviously contradictory to the real spectrum of light, as this is best represented as a straight line starting with low-energy red and continuing through to high-energy violet. There is no way to circle back to red from violet when speaking about the spectrum of light. However, we are able to construct the concept of a "color wheel" on the basis of how we perceive color, which is, in-turn, based on the three color receptors in our visual systems.

Mixing Colors isn't as easy as it seems...

FOR FURTHER READING

1. Rossotti, H.,Colour, Why the World Isn't Grey (Princeton University Press, New Jersey,1983).
2. Birren, F., Principles of Color (Schiffer Publishing Ltd., Pennsylvania, 1969 & 1987).