There's really not much I can say here, numberplates not being the most exciting product on our database, so I'll just explain for the new guys (AKA racing):
White, with red numbers = Midget/Rookies
White, with black numbers = Juniors
Yellow, with black numbers = Seniors
Red, with white numbers = Closed State Champion
Blue, with white numbers = Open State Champion
Green, with an Australian Gold Number 1 = National Champion!