For what its worth, I know that University of Jiu Jitsu (Xande and Saulo's school) have green belts between white and blue. I was talking to someone at the school about it and its really more for outsiders. I could be wrong but the gist of the discussion was that, they didn't want people who were just starting out as white belts, to feel like they were just getting absolutely demolished by other white belts. The green belt was sort of a belt that was given to say, this person is still a white belt, but definitely not a beginner and should be getting his blue soonish.
Yeah stripes work that way too, but I guess that its easier to differentiate with colors.