As Spat has said, green usernames indicate newbies.
Everyone who joins AB will have all of their questions and answers marked with a green username for the first fortnight. This helps regulars to be a little more forgiving of breaches of etiquette. For example, someone who posts a question relating to a crossword puzzle outside of 'Q&P' will probably be politely asked to use the correct category if their name appears in green. (If, however, their name appears in black, the response might be rather less polite :-) ).
Chris