Complementary colors are any two colors directly opposite each other on the color wheel. When mixed together in equal amounts, two complements form a neutralized, dull color. When two complements are placed next to each other in a pure form, they contrast and intensify each other.
The complements are:
Red and Green
Orange and Blue
Yellow and Violet
Each complement combination contains one primary color and one secondary color. So when mixing orange and blue together you are in fact mixing red + yellow = orange, then blue. All three primaries are present creating a neutral, dull gray, or brown color.
