Herald of Kozilek

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 {X} 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 {X}, you could choose 5 as the value of X and pay {4} 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.