It's probably best done from scratch, but on an existing guitar I'd guess black over white would give much better coverage, with just a few coats, than white over black. Then lots of clear coats over the top to get rid of the "step" between white and black.
The thing which always interested me was the scratchplate - presumably it's two plates, cut down the middle, then glued along the edges. Could be tricky to get that right.