As others had already mentioned, having a username in green means it's a new user which was created [I guess] less than a month ago. You may also see a green username from a story one year old which means that this user was new when he submitted that story. There is also a list of submissions from the most recent users created [1], also known as noobstories, comments from the most recently-created accounts are here [2] known as noobcomments.
1. https://news.ycombinator.com/noobstories
2. https://news.ycombinator.com/noobcomments