Tangential discussion:
I used to restore Wen-Mac Texaco Tankers from ~1961. I purchased sad old toys and fixed them up with new self-made parts, sometimes nearly as good as new.
The tankers had two white "houses" made from polystyrene, and I found a huge range of conditions on these parts. Some were hopelessly yellowed, some yellow on one side (I assumed they sat for years on a shelf exposed to a window), and some so white that they must have been kept in the original carton for 6 decades. But that whiteness ruled out simple oxygen exposure, it had to be the light.
What to do? The yellow was not deep, and some parts benefitted from light sanding and polishing. I also tried a popular remedy: Coating parts with hydrogen peroxide and exposing to MORE UV. Seemed to have some effect, but not enough to fix my parts. Usually the only solution was white paint.
And it affected the light gray parts as well. When I got started, I matched a gray paint to found parts- it was a gray with a tinge of green, which I used to paint 3d printed replacement parts. Later I found that the gray was UV affected too! Unexposed parts are a "pure" gray with no green tint. Had to get a new paint...
A badly yellowed example:
A nice example, un-yellowed after 60 years:
A restored tanker: