Color Theory Page

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Printers and artists have different definitions for primary colors. This document contains an analysis and a resolution for this difference.

• Introduction
• Additive Primary Colors
• Additive Secondary Colors
• Subtractive Primary Colors
• Process and Painting Primaries
• So how do I mix colors then?
• Conclusion
• Other sites

Introduction

Printers and artists have different definitions for primary colors. The traditionional primary colors that painters have used are red, yellow, and blue. Modern printing press secondary colors are magenta, yellow, and cyan. These two primary color systems obviously do not agree. In this document I will explain how the printing primary colors are derived. I will also show how these colors are related to the primary colors used by painters, and attempt to show how these colors are related.

Additive Primary Colors

Human vision relies on light sensitive cells in the retina of the eye. There are two basic kinds of sensors. These are rods and cones. Rods are cells which can work at very low intensity, but cannot resolve sharp images or color. Cones are cells that can resolve sharp images and color, but require much higher light levels to work. The combined information from these sensors is sent to the brain and enables us to see.

There are three types of cone. Red cones are sensitive to red light, green cones are sensitive to green light, and blue cones are sensitive to blue light. The perception of color depends on an imbalance between the stimulation level of the different cell types.

Additive color processes, such as television, work by having the capability to generate an image composed of red, green, and blue light. Since the intensity information for each of the three colors is preserved, the image color is preserved as well. The spectral distribution of the image will probably be wrong, but if the degree of intensity for each of the primary colors is correct, the image will appear to be the right color. Red, green, and blue are the additive primary colors, because they correspond to the red, green, and blue cones in the eye.

Additive Secondary Colors

Additive secondary colors are composed of two of the primary colors. These colors are shown in table 1.

Subtractive Primary Colors

Subtractive color processes work by blocking out parts of the spectrum. The idea of subtractive color is to reduce the amount of undesired color reaching the eye. If, for example, you had a yellow image, you would want to have a dye that would let red and green reach the eye, and block out blue. The additive secondaries become the subtractive primaries, because each of the additive secondaries will reflect two of the additive primaries, and absorb one of the additive primaries.

Additive Secondaries/Subtractive Primaries Absorption Chart

Color    Reflects        Absorbs 
Yellow   Red and Green   Blue
Magenta  Red and Blue    Green
Cyan     Green and Blue   Red

With this information, if we wanted red, we would mix magenta and yellow. Magenta would absorb green, and yellow would absorb blue, leaving only red to be reflected back to the eye. For black, a combination of all three would be used, which should block out all light in theory. Printers use black as well, since the dyes used in printing are not perfect, and some light from other parts of the spectrum gets through.

Process and Painting Primaries

True cyan and magenta have not been historically available. The blue pigments that are typically available reflect a certain percentage of green. This percentage is not as high as with true cyan, but it is still there. With this information, and using red, yellow and blue as our primary colors, table 4 shows what we could mix.

With the possible exception of certain pointillist techniques, process primaries cannot be mixed from other colors because adding different paints only subtracts from the light reflected back to the viewer. Painting primaries can be mixed from the process primaries. Since a primary color is one that cannot be mixed, the process primaries are the ones that can be considered true primary colors.

If the process primaries cyan and magenta were not available, (and for standard oil paint colors, this is historically the case) the next best primary colors to use would be red, yellow, and blue. Many colors can be mixed with these three colors and it is not possible to mix these colors without cyan and magenta.

So how do I mix colors then?

If you want to mix genuine paints there are a couple of strategies that you can use to mix reasonably clean colors. If you have a paint which is reasonably close to what you want, try mixing it with a color that is not too far away from it on the color wheel. If you mix with colors that are far away, you will tend to get darker colors.

Conclusion

Magenta, yellow and cyan are the true process primary colors. This agrees with what can be mixed, as well as with the additive primary color system. In the absence of magenta and cyan, red and blue can sort of be used as primary colors, although not all colors can be mixed.

Other References

The people at hunterlab have equipment that can do quantitative analysis of color, so you may want to check this out if you're looking for reproducable results.

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Ted Park, tpark@beer.org