Are you sure they are Mepros? They sound more like the older Trijicons from your description. The older Trijicons I've removed did not have the white ring, but just the dull aluminum vial. All the Mepros I've installed have had the white rings, and they have always been bright white (unless dirty). The older Trijicons also seemed to have a smaller diameter dot as well.
If they are Mepros, I'd call them and bitch, as they have a 12 year warranty on the green ones (and maybe some other colors). They should have a date code on them to confirm the born on date. I'd also bitch about the rings. As I said, mine have all been bright white (you do clean them, right?), and still are, even after ten years or so.