i've got a bunch of beads that are in shot glasses and the ones that have been in use the longest are yellowish. I assume that whatever is turning them that color is the same thing that turns cello yellow. the cedar maybe?
I'd have to lean against it being the oils from the cigars, as the beads are not in contact with the cigars and oil doesn't have the ability to change phases from a liquid to a gaseous state so it can migrate to the beads themselves, or at least not at the ambient conditions of 70 deg F and 65% RH. but what do i know??? just my .02