Good idea, best still would be to put the banners accordingly to your faction in Strategus. Your part of the faction, you are able to use it. If you are in no faction, you automatically use the default faction banner (Khergits for purple, Nords (the native faction, not the Nordmen) for blue, u.a.). Not sure if that was possible though. But it was suggested several times already.
bad thing about that would be bigger unions of clans though, i.e. crusader alliance (several clans are in strategus faction?, one banner)