Herald of Kozilek

• WWizards of the Coast2015-08-25
  Official Ruling
  Herald of Kozilek's last ability can't reduce the amount of colored mana you pay for a spell. It reduces only the generic component of that mana cost.
• WWizards of the Coast2015-08-25
  Official Ruling
  If there are additional costs to cast a spell, or if the cost to cast a spell is increased by an effect, apply those increases before applying cost reductions.
• WWizards of the Coast2015-08-25
  Official Ruling
  Cards with devoid use frames that are variations of the transparent frame traditionally used for Eldrazi. The top part of the card features some color over a background based on the texture of the hedrons that once imprisoned the Eldrazi. This coloration is intended to aid deckbuilding and game play.
• WWizards of the Coast2015-08-25
  Official Ruling
  Other cards and abilities can give a card with devoid color. If that happens, it's just the new color, not that color and colorless.
• WWizards of the Coast2015-08-25
  Official Ruling
  If a card loses devoid, it will still be colorless. This is because effects that change an object's color (like the one created by devoid) are considered before the object loses devoid.
• WWizards of the Coast2015-08-25
  Official Ruling
  The cost reduction can apply to alternative costs.
• WWizards of the Coast2015-08-25
  Official Ruling
  If a colorless spell you cast has in its mana cost, you choose the value of X before calculating the spell's total cost. For example, if that spell's mana cost is , you could choose 5 as the value of X and pay to cast the spell.
• WWizards of the Coast2015-08-25
  Official Ruling
  A card with devoid is just colorless. It's not colorless and the colors of mana in its mana cost.
• WWizards of the Coast2015-08-25
  Official Ruling
  Devoid works in all zones, not just on the battlefield.
• WWizards of the Coast2015-08-25
  Official Ruling
  Herald of Kozilek's last ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you actually pay.

Rulings via Scryfall