As I can see, every user with a submission on /noobstories is green, so that's probably the threshold. Edit: On further inspection, it appears I'm wrong, there

As I can see, every user with a submission on /noobstories is green, so that's probably the threshold. Edit: On further inspection, it appears I'm wrong, there are users there who are not green. The newest users I could find that are not green created their accounts 6 days ago. Users who created their accounts 4 days ago are green. I haven't had any luck in finding users who created their accounts exactly 5 days ago. So the threshold is either 5 or 6 days.