complementary

(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). Synonyms: complementary color. Example: "Yellow and blue are complementaries."

(adj) of words or propositions so related that each is the negation of the other. Example: "`male' and `female' are complementary terms."

(adj) acting as or providing a complement (something that completes the whole). Synonyms: complemental, completing.