complementary color

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Either one of two colors whose mixture in the right proportions produces white (in the case of light) or gray (in the case of pigment).
2. n. A color directly opposite another on a color wheel and providing the greatest chromatic contrast to it. See Table at color.

GNU Webster's 1913

1. n. one of two colors so related to each other that when blended together they produce white light; -- so called because each color makes up to the other what it lacks to make it white. Artificial or pigment colors, when mixed, produce effects differing from those of the primary colors, in consequence of partial absorption.

WordNet 3.0

1. n. either one of two chromatic colors that when mixed together give white (in the case of lights) or grey (in the case of pigments)