I thought the reason red got the kibosh was cadmium and other heavy metals (after all, red was popular and made until 99 while lead was gone by what, 1978?). Yellow I think had the same chemical issues and while it was gone in 78 as well, I believe that was due to a feeling it was dated/poor sales, not chemical composition.
I could be totally wrong, but I mean, I've read all that stuff on the internet so it must be true.