ahhh this age old topic again.
so far, theres no actual scientific proof that explains yellowing. some say sunlight, some say UV rays, some say humidity levels, some say it depends on the chemicals used to make the plastic, but no one really knows?
i had a MPC(dodges bricks) once and i had it boxed up in a closet away from all sunlight. after a few months i opened the box and BAM! yellowing on the wing! now this toy hadn't seen the light of day since the day i got it. i opened the box, saw how crappy it was, and i was ashamed to put it on display. so back in the box it went. there was no good reason for the yellowing whatsoever. no sunlight, no UV rays, little to no humidity, WTF???
some say, if your toy yellows, that direct sunlight will get rid of the yellowing but i have yet to see someone prove it. all i can tell you is try your best to keep it away from sunlight and and other outside factors(smoke) and your toys should stay good as new.