I just assumed green meant admin or some kind of paying customer.
I'd venture to guess green users could use this ambiguity to play subtle rhetorical tricks on users with a moderate number of points here.
your username will appear green till you get a certain amount of karma/upvotes.
I thought it was based on time.
hmm not sure. My username appeared green till i got 50/100 upvotes. I think that makes sense since the number upvotes on HN usually indicates that the person makes valid comments and is not trolling.