Here's a link to look at some finished ones.

http://www.google.com/images?sa=3&q=tray+ceiling+colors&btnG=Search+images

I am putting a tray ceiling in my MB..I was speaking with my decorator just today and she said to put up crown molding where the side wall meets the tray. Then she said to paint the walls a creamy white, paint the molding white dove (BEN M.) and then find a wallpaper in the cream tones..Farrow & Ball has great ones and wallpaper the ceiling and the angle sides. It sounds pretty so I think I will do that.
Good luck!

I have a tray ceiling (rounded) and there is crown molding as Coldny mentioned. All is off white except for the walls.

Although this is the same in an extra bedroom, all the trim, walls and ceiling are painted the same light tan. It makes the room look much larger. Not sure I would do this in every room though.

It would help to know what your style is, the colors in the room, the formality or informality of the design.