Within every color model, there are 3 primary colors from which all other colors are derived. In traditional color theory, the three primary colors are blue, red and yellow. In the RGB color model, the primary colors are red, green and blue. Any two primary colors combine to produce a secondary color. Tertiary colors are derived from either a combination of a primary and secondary color, or two secondary colors. We use color wheels to present all of these colors in a logically arranged sequence. As you can see from the illustrations below, an RGB color wheel differs significantly from the traditional artist's color wheel:

Painter's color wheel	RGB (additive) color wheel
Note the difference in the primary colors

Color wheels expose relationships between colors that can be used to achieve both balance and contrast. They often include a number of full-intensity (saturated) hues as well as a variety of tints, tones and shades, which are less-saturated versions of the hue that include more white, gray, or black respectively. While combinations of pure hues creates color harmonies that are very dynamic, you can create more subtle and subdued harmonies by using colors that are less saturated and closer in value-- that is, colors with similar degrees of lightness or darkness.

Some common color harmonies include:

		Analogous colors that are side by side on a color wheel
		Complementary colors appearing across from one another on a color wheel. These color combinations offer the maximum amount of contast but can be overstimulating if used extensively
		Split-complementary one hue plus the two colors on either side of its complement. Split-complement harmony provides less contrast than straight complements
		Triad three colors that are equidistant on a color wheel
		Tetrad two pairs of complementary colors

[fyi, for a great online color wheel that highlights these color relationships, check out Visibone's colorlab.]