I agree on the colorless theory. It fits the set. Lands do not need to tap for <>, this is already represented by (1) in the mana pool. But a cost that requires specifically colorless has never been done (maybe some random card?), it makes sense as a the <> symbol.
Edit: apparently there more spoilers and I have no idea what they are. So I'm wrong.
Edit edit: actually I just forgot mirrorpool.
Edit: apparently there more spoilers and I have no idea what they are. So I'm wrong.
Edit edit: actually I just forgot mirrorpool.