Formal: Contrast is a principle of art. When defining it, art experts refer to the arrangement of opposite elements (light vs. dark colors, rough vs. smooth textures, large vs. small shapes, etc.) in a piece so as to create visual interest, excitement, and drama.

http://arthistory.about.com/cs/glossaries/g/c_contrast.htm

Informal: the balance of black and white

http://facweb.cs.depaul.edu/sgrais/contrast.htm