It is apparently a percentage thing so you can have a very low post count and get a green name if you make mostly good posts
Actually that's the only time you CAN get a green or red name.
Well that answers it about the stars but not the titles... atleast the Scholar, Sage and Guru titles that are above the avatars.
Your title is an average of your karma across all posts. If your karma is above average across all posts, then after a certain post count (don't remember), you become a scholar and gain the ability to rate posts. If it keeps going up, you become a sage or a guru.
Because the karma is averaged across all posts though, the more posts you have, the harder it is for your title to change. For instance: I have nearly 2000 posts. It's probably impossible for me to become a guru at this point, just because each individually rated post will have only a tiny impact on my average karma, and that impact goes down the more posts I make.
Beyond scholar though, the titles don't really do anything other than pretty your name up a bit.