Devoid is a characteristic-defining ability that makes a card colorless even if colored mana symbols appear in its mana cost. A card with devoid is colorless in every zone, not merely after it reaches the battlefield. Note that this does not change the card’s “color identity” for Commander purposes.
Herald of Kozilek can reduce the cost of colorless spells, Ruination Guide strengthens other colorless creatures, and Ghostfire Blade becomes especially efficient when attached to the colorless creatures that devoid decks naturally produce.
