My usual method for painting white (if I want it to be brilliant white, not ivory or bone-like white) is to paint the area with that company who must not be named's "Ghostly Grey". I then do the shadows with a little more bluish version of the Ghostly Grey and then paint the highlights in White. It gives a fairly bright and rich look and the white really "pops" instead of looking dingy. This isn't what you'd always want, but on something like a brotherhood cloak, etc, it's given me some very nice results.