Blue Cavern

Is it sun? Or smoke? Or the way some of the plastic ages? My childhood smurfs that I bought in France back in 1979-80 are still white. But they are all Bully, so do the other plastics turn yellow?

Yes, it can be a number of things that turn the white caps into a yellow colour. From my experience it is mainly to do with nicotine smoke. This is not always to pick up when viewing smurfs online.

For the smurfs produced before 1997, especially the ones marked made in China : they have faded because of the sun, of the cigarette smoke or of acid used by people thinking it will clean the plastic.
For the other ones, I think the white plastic becomes easily yellow, just by the time and of course because of the same troubles quoted above.