It's an interesting quandary. On the one hand, you have a backlit screen, which as others have mentioned, has historically been surrounded by a dark frame, presumably, to my thinking, to replicate the classic darkened theater experience.
On the other hand, graphic design workspaces have traditionally been the domain of white; from the art boards to the desk surface, to the walls of the studio.
In reality, it shouldn't matter. Most of the better drawing/painting apps I've seen or used provide the user with a windowed/bordered canvas area anyway, which is often white, or sometimes user configurable. You're not likely to paint full screen anyhow...you need room for palletes. So if you use an app with an art board and a surrounding white work area, it shouldn't much matter what color your iPad bezel is.