I have a pretty goo Peru Starshine, myself (you actually almost got her in our trade, Nhal ;P ) so I took a closer look at her for you...she's more on the creamy side than yellow OR brown, so chalk that one up as a possibility as well. She's only *slightly* more white on the inside from what I could see. I'm pretty sensitive to color differences...so much that even a slight head/body mismatch drives me bananers.
Also noted she doesn't appear to have any cancer spots or other age related spots.
I think most ponies, no matter the color, turn more of a muddy, brownish color when exposed to too much UV light...yellowing seems to appear more as normal plastic aging/mictoscopic dirt, cigarette smoke...particulate matter that the pony is exposed to wear and tear. I think human skin oils have a lot to do with yellowing as well, since I've seen areas that are touched and held more often seem to yellow first (bumps on pegasus wings, ears and lips)
And I don't believe a Cherry Treats discussion should be opened without all parties having signed a waiver prior...damn Cherry Treats...
(and i agree with the scent/plasic chemical degradation thing)