[xattt]

I am not sure either. I have some quirky white plastic Matroshka measuring cups that I left out on the sill as a decoration for several years. They were exposed to sunlight, and have yellowed. Interestingly, if I run them through the dishwasher they become even more yellow, and fade with a bit of time.

I wonder if it has anything to do with sodium perchlorate in the dishwasher tabs, the heat of the dishwasher or both.