Grave Birthing

The target opponent chooses which card to exile as Grave Birthing resolves.

Eldrazi and Scion are each separate creature types. Anything that affects Eldrazi will affect these tokens, for example.

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

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.

You can cast Grave Birthing targeting an opponent with an empty graveyard. Even though that player won't exile a card from their graveyard, you'll still get an Eldrazi Scion and draw a card.

Eldrazi Scions are similar to Eldrazi Spawn, seen in the Zendikar block. Note that Eldrazi Scions are 1/1, not 0/1.

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.