I have only watched a few episodes of Being Human (the original BBC series) so I may have missed something. (Just got hold of the dvds so trying to catch up.) In episode 3, George transforms into a werewolf after/during fighting Tully in the shed. He exhales a green mist just after the transformation has finished. I didn't notice this in any other episode, did it have something to do with fighting Tully? Eg, proximity of another werewolf?
Any other explanation - maybe just werewolves have very bad breath?