If it's made of liquid-lame-type fabric, I have had that tarnish, too. The metal on the fabric actually oxidizes. As to how to clean it, I'm not sure if you're going to be able to do so without damaging the fabric. Probably the gentlest thing to try would be a soak in vinegar and salt. Looking online, another tip for cleaning metallic fabrics is sponging it with methylted spirits.
However, any of this may well completely remove the metallic paint.