I know that they develop a patina over time and I have, when excavating, noticed the pearly color you're talking about - it almost looks like they're drenched in solidified oil. But I have never heard of a color change. Very interesting.
More general on colors; I'm taught that a light Green was, from what I understand, and is, the original color of glass. Truly transparent pieces would therefor represent a higher quality type (as would other colors).