[tjr]

The green color indicates recently-created user accounts. I wouldn't purport to know why this was implemented, but I think it helps to identify potential spamming and trolling, as that tends to come more from accounts that were just created rather than from older accounts.

[opminion]

Colouring the author's username in green indicates that, most likely, the content of the comment has not been been influenced by the scoring system.

The scoring system serves two purposes: presentation, and feedback to the author. Feedback helps increase the quality of the future comments. Incidentally, forums with silent bans take advantage of this dual use, by changing one without the other.

[sp332]

It's also pretty inconsistent. Sometimes a username will be green and not-green on the same page.