As far As I can tell, it seems to be an oxidization of the plastic....from air touching it...either that, or it's radioactive and is decaying.!!
Anyone willing to smash one to bits to see if it is still white/grey where it breaks? If so, leave it out for another 5-10 years and get back to us on whether that turns yellow too.
Good luck, Mulder and Scully.
-Rob