New users get a green username: https://github.com/minimaxir/hacker-news-undocumented#green-...

Newly opened accounts. Often it's people who are regular users using a throwaway.

Interestingly most throwaway accounts I see with throwaway in their name aren't green, so I think those people must have just kept their "throwaway" account?

Yes they do, I'm doing it right now with this account. Notice the intentional misspelling (correctly spelled username is still available).

New account.