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.

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

Once you legally activate the ability, it doesn't matter what happens to the other colorless creature. The ability will resolve and create a regeneration shield even if you don't control another colorless creature at that time.

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.

Skitterskin's activated ability doesn't depend on Skitterskin being colorless. If it gains one or more colors, you can still activate the ability if you control another creature that's 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.

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