You can select certain faces and apply a new material to them. That's the simplest way, though there are more sophisticated methods (layered shaders) that might be more robust.
As for the ngons, those are not ideal, but as you are a new user, I would say carry on and keep exploring and don't get bogged down with elegant geometry. I would focus more on getting the shapes just right. Currently I feel that the dots are closer together than in a typical real die.