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.

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.

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.

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.

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.

The cost reduction can apply to alternative costs.

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.

A card with devoid is just colorless. It's not colorless and the colors of mana in its mana cost.

Devoid works in all zones, not just on the battlefield.

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.