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

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.

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

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

You can cast Adverse Conditions with no targets. When it resolves, you'll get an Eldrazi Scion. However, if you cast Adverse Conditions with any targets and all of those targets are illegal as it tries to resolve, it won't resolve and none of its effects will happen. You won't get the Eldrazi Scion in that case.

Adverse Conditions tracks the creatures, but not their controller. If either of the creatures changes controllers before its first controller's next untap step has come around, then it won't untap during its new controller's next untap step.

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

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.

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.

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.

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.