Good tupelo honey can have a green tinge to it, but I have never seen it bright green. Purple and red can come from elderberries if there are no nectar sources and the bees take elderberry juice from berries.
I'm gonna speculate and say that the most common cause of green in honey is from nectar that contains chlorophyll for some reason (plant stress or disease or strange plant), but not sure if that is what you have.
Another thing that comes to mind is polyethylene glycol (PEG) if Bubba left a bucket of radiator coolant sitting around or some spilled somewhere. PEG has a sweet taste and a fluorescent green. It is toxic to humans and dogs (tears up the kidneys as it is metabolized), but this is not always a quick effect - it can take days or weeks. Not sure what effect it has on bees.
Found this on another forum, a report of bees foraging off spilled anti-freeze:
i put antifreeze in my car the other day and accidentally spilled a bit on the driveway i notice a couple of my honeybees lapping it up. i promptly stepped on them and took the water hose and washed it away. has anyone ever seen honeybees go after antifreeze??? i'm sure if they took it back to the hive it would contaminate the harvest. just thought i would post this to see your thoughts about. http://www.beesource.com/forums/showthread.php?t=194230