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.

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.

If the creature an opponent controls survives the fight but would die later in the turn for another reason, it will be exiled instead, whether your creature dealt it damage or not.

If the creature you control becomes an illegal target (perhaps due to being destroyed by another spell or ability), Unnatural Aggression will resolve, but the target creature an opponent controls will neither deal nor receive any damage. If that creature would die later in the turn for another reason, it will be exiled instead.

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.