Last year I lit my spare room with a low-energy UV ‘sunlight’ lamp. While packing up my stuff to move house I noticed that many of my classic (OK, junk) computers had become slightly yellow on one side - the side facing the lamp.
I’ve not had much trouble with the old Macs (apart from the Mac Plus). The worst affected seems to be Acorn kit, even if it’s never been exposed to sunlight. Some of it turns a particularly foul I-don’t-want-to-even-touch-that colour.
This article at Vintage Computing explains the phenomena of yellowing plastic - it’s actually a reaction caused by flame retardants.