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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.

I think using green usernames for 'noobstories' is wrong. Green is usually linked to positive behaviour, earned reputation/karma or a special (empowered) user status; not for indicating that a user is new.

Green is also used to connotate inexperience or lack of maturity, e.g. greenhorn, green lumber.

Did they stop doing that already, or are we being A/B tested?

Still happening as far as I can see. Try /newest - there's more green showing there.