I've read similar info on medical websites. Fresh bile is yellow, while oxidized bile is green. It takes time for oxidation to occur, so it seems logical the green color probably indicates stagnant bile, and the darker green it is, the older the bile. The bacteria in the bowel convert the yellow or green bile into brown - which is why feces is brown by the way. If it isn't, then it means you have a lack of beneficial bacteria in your bowels.
On my first six flushes I got out many bright green stones with dark green interiors, giant yellow/tan stones with smaller dark green stones inside, and lots of yellow bile. I guess perhaps the bile was fresh but the green stones were not. On my last flush (the seventh), I got out dark green stones for the first time. I guess these were probably old ones then?