Now, I love black backgrounds more than anything else in the design business, and yet I was still very surprised to acknowledge how dark theatrical posters are and that, specifically, in this context, the top 25 grossing movies of all time across all ages didn’t run a very wide gamut. Only at the tot level did color start to play a real role. And while the psychological and emotional explanations of what colors mean are too varied to take any which one as authoritative, it is nonetheless telling that black is the color of choice in movie posters. —Chris Eichman
—Dark and Fleshy: The Color of Top Grossing Movies  (Under Consideration)

The top of the image represents NC-17 posters, while the bottom represents G movies.