yu gi oh zombie deck Synchro Undead 2012 / All cards will be carefully packed for safe transit. History of the Meta -

Additionally, in an elegant design choice, the cards that tap for two colors of mana have split costs on their front side and can be cast for only one color

(1) Zombie World(s) in the Field Zone cannot be targeted by, or destroyed by, card effects

Mortify is not as good against aggro, but also has application against control

The spirit of the rules would seem to suggest that these cards are the colors of both faces for deck construction purposes, but not only are these spells always cast as the front face, the front face is the only one 'seen' in other game zones: for instance, when the card's in the library, you could find Elbrus by searching your library for an artifact, but you couldn't find Withengar Unbound by searching your library for a creature card