Exerpts from my book, chapter on Color Theory ...

THINKING IN PRIMARY COLORS
Initially when an artist begins to match color by sight, there is much confusion about whether or not to buy all of those beautiful color mixtures, premixed in a rainbow of paint tubes, that are guaranteed to stretch your arm out when you carry all of them to a location beyond your own studio. However, if the creative mind can simplify its thinking by interpreting all colors into the three primaries, this can be a very easy process without creating monkey arms in the process. What I am suggesting is that you have in your palette, a warm and cool form of each primary color and a few other highly used colors like brown, white, and green. I will cover the specific colors that I use in the chapter on color (see page xx).
How do you begin to teach yourself to think in primaries? Look at each color and find the two significant primaries. If the color is bright and a red, yellow, or blue, but not pure as what comes from your tube, then add the compliment. The compliment is the color directly across from your color on a color wheel (see page xx). Think of your brown tones and gray tones as a mixture of all three primary colors. Both of these colors are created by adding all three of the primaries together. Understand that the only difference in mixing brown or gray is that gray has more blue, and brown has more yellow and red. Once you know the dominant color, you have to decide what the second color is that is influencing this tone. Next, think about secondary colors. Your job is half done if the shade is closer to an orange, green, or violet. Once you decipher the color into the two main colors, add the third primary and/or white in small amounts to dull the shade or tint to the tone that you are seeing. It’s that simple.