it seems to be a thing that just happens over time.
white model paint also yellows over time.
We have a lively and long-running discussion on the subject.
for me: all of my yammies are exposed to indirect [and sometimes direct] sunlight every day, a couple for 3+ years and they're fine, so far. All have been purchased 'used and displayed' by their previous owners, from various members here and elsewhere, none are yellowing on me.
[I only have 1 DX valk [VF-31C] and it's simply not old enough to be factored into the discussion as evidence.]
Everyone's mileage seems to vary, so zoom them around the room/play with them regularly while they're pristine, and that way you won't notice the gradual shift as much [this is my plan btw ]