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.
The color wheel also helps you identify warm and cool hues.
Half of the color wheel, from red to yellow-green, is considered warm, stimulating, and advancing. Such a description reflects emotional associations (the sun looks yellow, and fire is orange and red, for example), but it has a basis in physiology: The eye can't bring the red and purple ends of the spectrum into focus at the same time, so it perceives red to be nearer or advancing.
Tip: A warm color scheme needs a dollop of a cool hue to feel well-rounded and complete; think of a green plant in a yellow room.
The other half of the wheel is described as cool; these colors generally appear to recede. Thus a small room may benefit from visually opening up the walls with a cool, or receding, paint color such as blue, green, or purple.
Tip: A cool scheme needs a jolt of warmth to liven it up; thus a shot of red will perk up a room done in blue and white.
Tip: Green and purple may seem to either advance or recede, depending on the context; for that reason, some interior designers consider them neutrals that can go with any color.
You're probably attracted to colors not only for their specific hue -- red, blue-green, orange -- but also for particular values of those hues, such as pink, teal, or terra-cotta, for example.
Value refers to the lightness or darkness of a color. A hue's value becomes lighter with the addition of white; black or umber (a blackish brown) darkens the value. Sky blue and robin's-egg blue are light values of blue, while navy and cobalt are dark values.
Balance with Accents: Light and medium values live most comfortably with each other, but to keep a light-value scheme from becoming boring, include an accent of a darker value. In a room decorated in light blue and light yellow, for example, a touch of navy blue or cobalt blue will ground the scheme and give it depth.
Another aspect of any color is its intensity or saturation. The pure hue represents the most intense or most saturated expression of a color. Adding the hue's complement will gray or muddy the color so that it's softer, more muted, and less intense.
Lower-intensity colors generally create a calm, restrained mood that's subtle and serene.
Higher-intensity (more saturated) colors generate more energy and can feel dynamic or richly elegant, depending on the specific colors and the style of your furnishings.