taken from Basic Colour theory on http://www.colormatters.com/color-and-design/basic-color-theory
Some Formulas for Color Harmony
There are many theories for harmony. The following illustrations and descriptions present some basic formulas.
Analogous colors are any three colors which are side by side on a 12 part color wheel, such as yellow-green, yellow, and yellow-orange. Usually one of the three colors predominates.
2. A color scheme based on complementary colors
Complementary colors are any two colors which are directly opposite each other, such as red and green and red-purple and yellow-green. In the illustration above, there are several variations of yellow-green in the leaves and several variations of red-purple in the orchid. These opposing colors create maximum contrast and maximum stability.
3. A color scheme based on nature
Nature provides a perfect departure point for color harmony. In the illustration above, red yellow and green create a harmonious design, regardless of whether this combination fits into a technical formula for color harmony.