You know, I never really realized that before, especially since green is the colour that makes us feel relaxed and comfortable.
On the subject of mold: mold itself isn't inherently bad. Basically, it's like the process which mark12_30 has pointed out about the grass.
Bilbo's door is green, and he was a good guy. Of course, you could also refute that point by saying that Bilbo was the Ringbearer of Sauron's Ring, which had an effect on him, making him feel like butter being stretched over to much bread, etc...
So maybe green is an ambigous colour, used by both bad guys and good guys. The Rohirrim, also, had green banners.
Half formed thoughts here...like mark12_30 said, more later.