Nice built-ins. I like your idea of the back wall a darker color and the shelves a lighter color. You might consider painting the walls a light neutral color like gray, the shelves white, and the back wall either a darker shade of the neutral wall color, or a more vibrant color if it suits your taste. I know a lot of people have strong feelings about painting over brick. I would do whatever you like best, just make sure either the brick or paint color goes with everything else. Here's a pic from Young House Love on a similar project--I'm sure you can find some other good examples on Houzz.