"Fresh" bile that hasn't had enough time to be converted. Usually, anyway, if there is a "problem" in the gut. Generally it means the foodstuff has moved too quickly through the intestinal tract. Just think this way: the bile is excreted right after the stomach, way up in your system. It's green. If it rushes through the GI system so it doesn't all get converted, alot can stay green.
Well i vomitted up on an empty stomach something i could only describe as being like a slightly thick uring coloured liquid, im guessing this was bile, i had a reaction to antibiotics too many on an empty stomach i DONT recommend it !!
Keep an eye on your stool, and go to Docs if if persists,