I remember an episode of "Top Chef" where the contestants were asked to put together a menu centered around a color. If I recall correctly, the person who got green botched the opportunity. It's a very pervasive color in cooking, and should provide the widest range of ingredients.
I had thought about having a "Black" dinner, with primarily black ingredients - squid ink, black sesame seeds, pidan, black garlic...truffles (grin). But the real challenge is the color blue. Mainly because there really isn't a naturally occurring blue pigment in biology - in most cases where we perceive blue, like in butterfly wings, it is primarily some kind of prismatic effect. It's so rare that humans naturally reject blue coloration in food - give it a try. When food is colored blue, it seems to become less appetizing.
Okay, someone is going to say blueberries - which are really purplish black. And blue cheese - and not everyone thinks that appetizing :).