I've acquired a silver tea service that I'm going to clean up and sell for a friend. I used Wright's tarnish remover for silver and copper. The label on the tea tray says it's copper covered with silver. When I remove the tarnish, it leaves a yellowish stain behind, as if there's tarnish that I didn't get off. Is that tarnish or did I damage the silver? Thanks for any help you can give me.