The green gun was released early on in the early bird sets. I wouldn't say that it is a discoloration its a dark green plastic. It is documented in the collectors archives and I can say with 100% certainty that they are out there. They are uncommon but not all that difficult to find. I am unsure if any made it to carded figs I don't beleive any did.
There are a few versions. lip color, bag color and so on. As for the green limbs, that seems to still be up for discussion depending on who you ask. I count it as I feel it did not happen at all the plants cranking out the chewie. I may be wrong on that. Thomas has a few oddities, 1 was discussed previously here as I was the former owner. I will try to dig up that thread.