BTW, you probably already know this, but sun is not the only culprit of discoloration. In fact, a chemical known as Bromine is often the cause. Bromine is a flame retardant that was added to most plastics in the 80's and 90's. It is a brown color, which is what causes most of the 'yellowing' you see on the white figures. Perhaps Bromine was added to the actual packaging/bubbles as well?
That said, UV light does act as a catalyst for the chemical change that brings the Bromine atoms to the surface.