Catacomb Sifter

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

Sacrificing an Eldrazi Scion creature token to add {C} is a mana ability. It doesn't use the stack and can't be responded to.

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.

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

Some instants and sorceries that create Eldrazi Scions require targets. If all targets for such a spell have become illegal by the time that spell tries to resolve, the spell won't resolve and none of its effects will happen. You won't get any Eldrazi Scions.