It really depends on the quality of the plastic being used. The yellowing, while generally very strong in the sunlight, can still occur with ambient light, as most sources will still emit light in the wavelengths that can cause this damage. As an example, think of some of the original Star Wars action figures (or something similar). Many, many of vintage blister packs suffer from similar yellowing, which is also accompanied by even worse brittleness (both are due to oxidation of the plastic). I have seen plenty of yellow Stormtroopers in my day, and I am sure they weren't all spending time on the beach.
Unfortunately, the process is irreversible, and is just something I am afraid we have to live with.
But then again for some reason my clear pink steven is extremely tall like he got stretched getting pulled from the mold.
