The color wheel arranges color around a circle according to similarities and commonalties. The colors on the wheel beside each other are the most related. For example, the orange is most similar to yellow and red. However, directly across the wheel from orange is blue. Blue is the complement of orange. Complementary colors are the colors directly opposite from one another on the color wheel. 

There are only three complementary relationships or pairings.

Green is the complement of red.

Orange is the complement of blue.

Purple is the complement of yellow.