complementary

This means something adds to, or makes better in some way, something else. So I could say, for instance, that someone’s top complements the colour of her hair – meaning it is better than just ‘in harmony with’, it makes the hair colour/style actually look better. Another example would be two tastes which, when combined together, make a better flavour than either could on their own – each on their own seems to be lacking something, but the other taste adds what is missing.

Should not be confused with complimentary, which is giving praise – the comment about someone’s top complementing the colour of her hair is a compliment.