Purple royalty and Red warriors seem to be the few themes that were around since the start. In the Kings expansion, since so many kings were being printed, it probably seemed a bad idea to put them all in one brigade (just as in Warriors, the warriors weren't all in Red), so they "classified" them into the various brigades ("well, Jehoshaphat went to battle, let's make him Red"). In Priests, several of them were reprinted into Purple, leaving only a couple like King Asa unreprinted.
I don't know why King Solomon wasn't made Purple though. Maybe because Wisdom of Solomon was already in Gold?
