The premier Spanish Magic resource magicmadrid.es has an interesting post saying that a reliable source has told them that the set will be entirely composed of two types of cards, either colorless cards (such as Kozilek, Butcher of Truth and Ghostfire) or of hybrid cards (such as Beseech the Queen.)

A theory suggested on the MTG Salvation forums says that the hybrids may be colorless/colored such as the example Beseech the Queen. This would definitely be an interesting idea to add to the compilation of Rise of the Eldrazi knowledge but it is still 100% rumor.