Ok first thing you have to know when you're drawing metal, is that you only draw the reflections or only the lighting, but because metal is very reflective there is a lot of constrast, try using a black/white gradient and draw a few reflective gradients on top of it using a fully transparent secondary color, experiment with the value, but stay in the black, grey, white area, you'll start to find a metallicish look, go for light colors mostly. Here's an example, look at the color window.