http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=33929
I ran a little test a few weeks ago with some older hasagawa decals that had yellowed a bit
I posted my before and after scans in my last reply in the thread (link above)
check it out as I was amazed however fast the sun took out the yellow (like 2 days and then after 3 or 4 no yellow at all) Decasls were placed in a Ziplock bag and then put in good south facing window.
I will also link the images below too
Below is the starting scan (2pm Tue) and a scan only a little over a day later (5pm Wed ).