For one of the best descriptions of what is going on:
http://www.doublegunshop.com/doublegunjournal_v7i4_9.htm

This is an article originally published in the Double Gun Journal by Oscar Gaddy. He has SEM images of the surface which results in the colors. Traditionally, after the surface is re cased, it is coated with a light film of lacquer to protect it.

Pete