IIRC your username would change color when you would reach a certain amount of posts, now you can change it anytime. Some users never changed theirs, so they have the default color which changes to grey when banned. If you decide to change the color, it'll say that way regardless.
And I don't think there's anyway to tell who's self banned or were by the mods. There's a user discussion thread in off topic and that's where you can ask questions about a user who isn't here anymore and what may have happened to them.