I have noticed that small amounts of avgas in pots goes green - and it happens with new avgas too. I use the stuff from my sampling pot in my mower. Incidently, I don't leave the delivery pipe full of fuel as it seems to go green in the pipe if left for a few weeks. On the basis of this I dont think you have a problem with the fuel in your bowser.
I take the view that if its blue and crystal clear out of the pump, Ill burn it in the aircraft. The stuff I have now is about 9 months old, and it still looks good.
I don't know if its light, evaporation or oxidation that makes it green but I suspect it is the loss of some volatile componant.
Try the same experiment with a sealed vessel if you are that bothered.