complementary color

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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).
• noun A color directly opposite another on a color wheel and providing the greatest chromatic contrast to it.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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