The complement of a color is the color you must add to it to get a neutral gray. You can find the complement of a color by locating the point on the color wheel an equal distance on the other side of the center.
The following primary colors are complements of each other:
Red and Cyan
Green and Magenta
Blue and Yellow
The following primary colors are complements of each other:
Red and Cyan
Green and Magenta
Blue and Yellow