Black/white plates are normally A class riders, yellow plates are B class riders, and C class guys run what they brung most of the time. A Class would be your pro/ams, B class are your intermediate riders that are a little slower (normally) then the A class guys, and C class is the Jr Class, normally C class would likely be the larger of all the classes.
You can have "pro" black/white plates and still be an amateur. Meaning you are fast enough and have scored enough points to be bumped out of B. But you pay your own way, or with some help from some sponsors.