I've done a lot of research myself on painting trays. I've seen the walls painted the same color as the walls of the room and the upper (flat) portions painted white or at least the same color. Someone also did the top part in beadboard wood which they stained. It looks good and it isn't too difficult to install.
I also saw a ceiling with the walls and flat portions painted a darker shade than the room walls and accented by white crown molding. It looks really elegant. I might do mine this way.