A lot of models I've seen use bright, polished looking brass paint to represent fired shell cases. They are not terribly bright as they come from the factory, and firing darkens them a bit. If they've been lying around a couple of weeks, they will be a dark brownish color. Also, some ammo uses steel cases which are lacquered to prevent rust. These are brown to start with, and rust fairly quickly after firing. Fired cases are somewhat soft, and will often be crushed or bent or dented if they've been stepped on by soldiers or run over by vehicles. If you use a brass or gold paint to simulate the case color, I'd recommend you dull it a bit with a tan or brown wash or overspray.