I was just looking at a digital painting one of the artists I watch at Deviantart uploaded today. What caught my eye more than anything was not the image itself but the copper armor they had given the character. I am always impressed with someone who can create metallic looking surfaces with non-metallic colors. I will admit that trying to capture that effect is constantly on my mind, scampering from one dusty, cobweb filled room in my head to another.
The tricky part is finding the right highlight and shadow tones to mimic the natural light reflection of the metal. Sometimes you can use white for a really intense light reflection but more often than not you will need a pale tone of the main color. Also, depending on the lighting, you may want to bleed some of the color out around the metallic part of the image to capture the reflected light off the item. You will show the glow of reflected light that way.
However you achieve the effect, a well done lighted metal item will draw anybody’s attention to your artwork. The best thing to do is just try drawing as many metal images from real life items as you can and see what happy accident lead you in the right direction.