first off you need to have a medium that you have a good understanding of. Enough that you know how to produce certain textures. if you haven't yet then no tutorial will be of use to you. but if you do, then copy as much real life photos that you an. its the best way to learn how to color metals. I would be doing this right myself now but i lack a significant amount of motivation, so I hope you have a lot of that laying around.