I don't know how a head would get rid of shininess without some other strange implications, because it is tied to the color you are using I believe. It is why it seems, when you use the expanded color palette for hair, it doesn't appear to be shiny.
What causes it is transparency, except instead of being transparent it picks up an environment map texture, which gives it a shiny appearance. Same goes for why you see strange shiny borders on certain things, because where the shiny texture is, is supposed to be 100% transparent, but it wasn't set up correctly or NWN doesn't agree with your mortal desires. '>
Beyond that, I don't think I can be of any help.