Use the 60-30-10 Rule
Decorating a space in terms of color is as easy as 60-30-10. Don't believe me? Take a look at some rooms in magazines or in Designers' Portfolio. You'll notice that the rooms you like the most are almost invariably divided into percentages of 60-30-10. Why this works is anybody's guess. Perhaps it is the human tendency to see an overall theme in the 60 percent hue, unifying the coloration. The 30 percent provides visual interest and the 10 percent, not unlike jewelry, provides that little spark of sparkle.

So, when decorating a particular room, divide the colors into percentages:
60% of a dominant color
30% of a secondary color
10% of an accent color

When you think about it, this color breakdown is similar to a man's business suit:
60% of the outfit's color is the slacks and jacket
30% of the outfit's color is the shirt
10% of the outfit's color is the tie

Translated to a room setting, it typically means:
60% of the room's color is the walls
30% of the room's color is the upholstery
10% of the room's color is, say, an accent piece or a floral arrangement