Anyone know why? I'm curious... the internet doesn't offer much, but this seems to indicate it's an insult to have anything but Blue and White: http://answers.yahoo.com/question/index?qid=20080504174420AA0kz1N because those are the colors of the Bavarian flag
From http://www.toytowngermany.com/lofi/index.php/t59808-15.html
it used to be red and white as it was the symbol for EMW, Eisenach Motoren Werk, which is where the factory was before the war and changed to blue and white when It became BMW and moved to Bayern and yes it was supposed to represent a propeller.