Yeah, it's not your fault man.
This happens - and very little other than storing them in a perfect lightless vacuum is going to make much of a difference. The box didn't help, but even white figures that I have kept in the storage plastics in dark closets (thus not enjoying them at all) have signs of yellowing.
So what do you do now?
Read this:
http://forums.yojoe.com/toys-1982-1998/ ... found.htmlI haven't been able to test this myself, but it's one of the things that I have wanted to do for a while. I've read many endorsements to the effect that this is the way to go.
-R