The color wheel offers the easiest way to visualize how hues relate to each other. Traditionally, artists have defined red, yellow, and blue as the three primary colors from which all others on the wheel can be mixed.
Although this is technically true, an artist can't actually derive a pure green or purple from the primaries -- the intensity of the mixed color won't equal that of the parents.
For decorating decisions, however, you need only be aware that purple relates to both red and blue and that green derives from yellow and blue. Those relationships mean the colors will harmonize with each other.
Reading the Wheel: The color wheel generally shows the pure hues of colors: red, blue, and green. In decorating, however, you're more likely to be using tints (lighter values) and tones (also known as shades) that are darker values of a color. For example, you may not use an intense green in a room; you're more likely to go with a soft sage or a deep hunter green instead.
Colors that lie opposite each other on the wheel are complementary; when paired, each makes the other appear more vivid.
Hues that lie beside each other are analogous; they always look good together because they share a common hue.
Triads are any three equally spaced colors on the wheel. These yield a lively yet balanced combination, but the scheme may feel a little jarring unless you let one color dominate and use the other two in lesser amounts or as accents.