The names of new site members are shown in green for the first fortnight.
The official reason is that it alerts us 'regulars' to the 'newbie' status of a poster, so that we can more easily forgive errors (such as posting in the wrong category) and guide the new member towards the best ways to use this site.
In practice, particularly in Chatterbank, a green username flags up a warning that the contributor might be one of those people who're continually getting banned (and thus constantly requiring new usernames).
Chris