When I first got my refurbed plum bolt SVT I cleaned the bolt and carrier by soaking it for a few hours in a container of "Ed's Red" homemade gun cleaner. To my surprise the plum colour started to come off when I was wiping it down. Surprisingly little effor with some fine steel wool took it all off in about 5 minutes. I was a little worried that I was ruining the rifle but now I've come to the conclusion that it looks nicer and does absolutely nothing to the value of the rifle.
I think it's a side effect of long term exposure of that type of steel to the storage grease but I have no evidence to back that up.