On a color wheel, complementary colors are the colors that are positioned directly opposite to one another. Common pairs of complementary colors are red and green, blue and orange, and yellow and violet. By combining two of these colors, you can create stark contrasts and visually exciting designs, provided you don't oversaturate colors. Doing this can be jarring to the viewer.
The color wheel is used to choose color schemes, such as complementary colors, split complements, a triadic scheme, or a rectangular scheme. Each of these have different uses and effects on the viewer.
A split complement design uses one base color and the two colors directly adjacent to its complement. For example, if blue were the primary color of a design, orange-red and orange-yellow would be the split complements because orange is the direct complement. Split complement schemes provides the same visual contrast without the tension.
Triad schemes use a set of three colors. One dominates and the other two provide accents. It's important that these colors be equally spaced along the wheel.
Even if you don't have a physical color wheel on hand, computer graphic design programs frequently have one that you can reference or possibly input a color and find its exact complement.