While trying to balance my weapons skills as a young Warrior, I spent a LOT of time hitting blues, on the theory that a hit is a hit and eventually your skill will increase. On one occasion I did get a skill up on a green. Did happen, for sure, only ting I was hitting. When I posted this, it was suggested that if, as you entered a level, a MoB is lt blue, but as you are moving up that level it becomes green, there is a slight chance that you will get a skill up killing the now green MOB. Don't know if that is what really happened, and haven't seen a green skill up since, but there it is.
Fineman 30th Warrior

