13 letters · 24 points in Scrabble
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)
adjective of words or propositions so related that each is the negation of the other
adjective acting as or providing a complement (something that completes the whole)
originFrom complement + -ary. Piecewise doublet of complimentary.
Etymology from Wiktionary, CC BY-SA 3.0