from my experience of nmm (i\'m crap at it), you need really good blrnding to pull it off. also, bear in mind that contrast is important in illustrating how shiny the metal is. ie if it is really polished, the colours would go from nearly black to white. but if it is dull, the shades would be similar.
hope that helps. there are others (ie most people on the site) who are better qulified for answering the question than me who should be able to give more thorough help