You had me wondering, so I just double checked some of my earrings. I have newer white gold earrings with the disk back and older yellow gold earrings with the Y back. So I think the best explanation is production batch/year instead of due to the gold color. It’s confusing, because there is a big overlap (I have earrings purchased the same year that have different types of the backs).
You’re 100% right- the y back are older, although they were around for a while if earrings weren’t sold….