Christopher Nolan discusses this topic in the commentary track of the DVD:
One of the reasons that I wanted to shoot it black-and-white was that it suited the type of, very sort of, film noir kind of story that I wanted to tell. But I also knew that having to light with very few lights, very quickly, if I was able to use this hard light look, with hard shadows, that I decided early on that I could achieve a more sylish look quickly - no frills. Whereas shooting color, I think, at that budget level where you just have nothing, and you have literally three 1K lights and maybe one bigger light. It's very tough to do these kind of nighttime interiors. So whereever possible I tried to use the lamps in the background, just really throw out some interesting shadows. But the extremity of this style of light, harking back to film noir and all the rest, is a lot easier to achieve in a very sort of quick and hurried and no frills fashion than I think a similarly stylish look would be possible to achieve in color.
