There's a link at the bottom of every page page which answers this, as well as other frequently asked questions:

https://news.ycombinator.com/newsfaq.html

(It's also the first hit for me if I Google [Hacker News green username].)

I believe it has to do with the fact that it was just registered today

Green means a new user - if you’re verified your name appears as everyone else’s but it will say “dang”