This obviously wouldn't help a new message being sent to a user (
in your example earlier), or a new post/reply (action=post)... those names all sit inside of text fields with no way to apply color coding effectively, especially if factoring multiple recipients (in messages).
You are correct - applying a font color would affect the entire textbox. You'd have to insert a new row below the textbox, break it into cells the size of each username and then highlight each cell border with a color that indicated if it was a newbie account. Could be done in javascript - send me a PM if interested.