We used to make a guard. Basically a bit of fine shim wood or card with a slot in it that went under metal studs and buttons to clean them without getting the brass cleaning stuff on the leather or fabric.
One of my handbags which has brass studs went green too but the only way I could clean those studs since they were in so tightly was to made a cardboard piece with a hole just hte right size cut out of it so that it fits snuggly around the studs.
Bit of a footer, but it works and the light tan leather bag is unmarked.
Any other ideas gratefully received
cheers,
Toddy
Ooops, too slow