If you want to be technical about it (you don't), this is perhaps one of the only franchises where this could possibly make sense, as it takes place in the far future where society has gone medieval after a massive cataclysm but prior to that it was an advanced society probably full of melting pot cities.
When the shit stopped working and people went back to making wood cabins and tilling fields, it's probable they stuck around and that'd explain the various colors even in small, isolated communities.