The complementary color of a primary color (red, blue, or yellow) is the color you get by mixing the other two primary colors. So the complementary color of red is green, of blue is orange, and of yellow is purple.
What About Secondary Colors?
The complementary of a secondary color is the primary color that wasn't used to make it. So the complementary color of green is red, of orange is blue, and of purple is yellow.
Why are Complementary Colors Important in Color Theory?
When placed next to each other, complementary colors make each other appear brighter, more intense. The shadow of an object will also contain its complementary color, for example the shadow of a green apple will contain some red.