In art, adding contrast to a piece would be putting together things that are . alike
not alike
similar
identical

not alike

Explanation:

The contrast in art means combining two very different things that are likely opposing each other. It is most often used for describing color contrast (light and dark, bright and dim, etc), but it also concerns contrasting elements (big and small, sharp and oval, etc.) or techniques.

This method is one of the main principles of designs and is used to give life to the art, make it more interesting and vivid, as well as to add variety. The contrasting elements often pop out of the art piece and bring the attention of the observes, so it can be used to direct the view of the person who is looking at the piece.