(As a Siege enters, choose an opponent to protect it. You and others can attack it. When it's defeated, exile it, then cast it transformed.)
When this Siege enters, exile target nonland permanent an opponent controls that isn't exactly two colors.

Whenever you cast a spell that's exactly two colors, look at the top six cards of your library. You may reveal a card that's exactly two colors from among them and put it into your hand. Put the rest on the bottom of your library in a random order.