Airbrush artist Dru Blair developed Color Buffer Theory in the late 20th century and has continued to develop his ideas since then. Blair's theory is that to obtain the precise shade and tint artists want, they should use white paint to "buffer" the darkness and intensity of the color paint they're using.
The Color Wheel
The artist's color wheel--named the Munsell wheel for creator Albert Munsell--places complementary colors such as red and blue-green or yellow and purple opposite each other on the wheel. Theoretically, if paints of two complementary colors in equal amounts were mixed together, they'd cancel each other out and make gray. Blair's website states he uses a revised version of the wheel as the basis for analyzing and buffering colors.
Novice airbrush artists often use pure colors fresh out of the paint bottle, Blair's website says, which is a mistake: Colors in the real world are almost always "contaminated" by other colors, graying them, so pure color doesn't look real.
It's possible to buffer one color by mixing in its complementary color, Blair states, but it's difficult to get the shade exactly right. Mixing in quantities of white mixed with black--in other words, gray--allows for more subtle, precise changes in the hue an artist wants. This is particularly important with transparent paints because multiple layers will make the paint less transparent and much darker. Buffering transparent colors with white, according to Blair's theory, limits how dark they can become.
When buffering colors, there's no need to guess whether you've added the right amount of buffer, Blair's website states. Make a test spray first and see how your buffered mix compares to the color you want. If it doesn't work, add or reduce the amount of gray. Blair recommends using the back of a photograph for tests because it's white, bright and won't absorb much paint.
When the colors are sprayed through the airbrush, Blair states, painters frequently see "color shift," making everything bluer. The reason is that as the paints interact, some of the colors bond and separate from the bleach used in white paint. Blair's solution is to use orange--the complementary color to blue on his wheel--to cancel out some of the bluing effect.