Colorless Prison wants to lock you out with Trinisphere and grind you down with Urza's Saga, The One Ring, and eventually Ugin or Emrakul, so speed and disruption both matter here. Unmask is excellent since it's uncounterable and lets you strip a Trinisphere, The One Ring, or Karn, the Great Creator before it locks up the game, and you'll rarely need the card you pitch. Kiora's Dismissal can bounce a problematic The One Ring or a freshly animated Urza's Saga token-producing permanent to buy a turn. Cut clunkier or redundant cards since you need to close the game fast before Trinisphere or Ugin takes over.