When people say you're green, its meant as a kind of insult. Though they're trying to say you're naiive or inexperienced, I can't help but see it as a good thing. Being green does mean you don't know every little thing...but it also means your opinions aren't hardened, you're not cynical or jaded. One who is 'green' can see the vast possibilities since they don't know what to expect. Endevors are new and fresh and, well...green like early spring.
The challenge then is to be green even when you know it all.