The exhausts were normally a rust colour, maybe with a bit of metallic sheen here and there. For now, keep it simple. Use the Dark Earth (brown) of the camouflage topside colour, and add a spot or two of red, until it looks like a brick red or rust colour, and paint the exhaust pipes in this. It should look fine. If you want, when it's totally dry, rub a pencil over the pipes to give just a hint of metal. But that's a long way off yet, after all the main assembly and painting has been done.