What is the purpose of constructing a color wheel? Simply put, to set in your mind the structure of color and how each is made. The three primary hues -- yellow, red, and blue -- are the basic building blocks of color. All other colors are derived from these three. The three secondary hues -- orange, violet, an green -- made of primary colors. And then the six tertiary hues -- yellow-orange, red-orange, red-violet, blue-violet, blue-green, and yellow-green. The color wheel has a total of twelve hues, arranged like the numbers on the face of a clock.
More on color theory: http://swegenas.aurora.esu9.org/midart/color_theory.htm