Here is a short step by step showing how I render metal! Before rendering you should know the shape of the metal. If it's curving or pointing out. The light that hits the metal's shape will alter how you can shade it. In this case I show metal that curves around the arm. I always use a combination of cool greys, browns, orance and pale bleak yellow. In this case the brightest point of the metal is in the middle of the armbrace, the color will darken near the edge of the armbrace before brighter colors tint the very edges. The color on the edges is lighting that is reflected to it by the environment. I top it all off with an overlay layer with some soft red/orange light. Hope this helps a bit if you've wanted to draw metal!