It's based on number of posts and average rating of all posts. I personally wouldn't worry too much about it at this time as people are still getting used to "rating" posts. But basically, orange = low post count low rating (as best I can now tell) yellow = high post count low rating, blue (darkish) = low post count, average rating, light blue = high rating (don't think post count matters too much), red = admin (if bold) or redpill if not bold (but I still don't know what the redpill rating is).
