Complementary colors are located directly opposite from each other on the color wheel.  (Follow the black lines that go through the center of the simple color wheel.)  For example:

• Blue complements orange.
• Green complements red.
• Yellow complements violet.

Because the secondary color in each complementary pair is made up of the other two primary colors, you can think of these combinations as having all three primary colors.  The complements "complete" each other.

When used together, complementary colors intensify each other. Red flowers, for example, look brighter against a green background.

The red foliage of the coleus makes the green moss stand out.

When yellow and purple flowers bloom in close proximity, the whole effect of these complementary colors is greater than the sum of its parts.

Blue looks more striking in close proximity to its complement, orange.

next: Analogous and Adjacent Color