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

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.

If that player has one card in their library when Sire of Stagnation’s triggered ability resolves, that card will be exiled. If their library has zero cards, no cards are exiled. In both cases, you’ll still draw two cards. That player won’t lose the game (until they attempt to draw a card from an empty library).

The cards are exiled from the library face up.

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

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.