Hi, Joy! Interesting question!
In a sense, all green things grow from something else that is dead or decayed. Without leaf mulch, your green trees wouldn't be green. Without decaying grass, the new grass wouldn't be so healthy either.
Forests are green, but good.
Fields are green, but good.
Swamps and meres are green-- but icky.
Underwater moss is green, but icky to step on.
Rotting food is green, and definitely icky.
I think I'll stop there for the list.
What other colors in LOTR are ambivalent?
How about black? Isn't it odd that the Guards of the Citadel are robed in silver and... black? And the tower of Orthanc, whick was built by good guys, is black.
How about White?
Gandalf the White is good.
Saruman the White is slipping fast. The White Hand is bad.
Minas Ithil started out white and good. Now it's green, sickly, yuck. And (interestingly!) there are white flowers in the river that comes out of Imlad Morgul that are evil, stinky, nasssssty. Bad flowers??? Hello???
Red Flame is ambivalent too: Narya is good; Orodruin is bad. Flaming Roggie is bad (Oh, how about just misunderstood and persecuted??
)
But back to the Green...
Food for thought. But I'm wondering if the key isn't in the concept of good vs bad decay-- perhaps unnatural decay or wrongful decay? Poisoned decay? And therefore contamination. More later.