塵の中を忍び寄るもの

A card with devoid is just colorless. It’s not colorless and the colors of mana in its mana cost.
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.
Like most abilities, Dust Stalker’s last ability functions only while Dust Stalker is on the battlefield.
Dust Stalker’s last ability checks to see if you control another colorless creature as the end step begins. If you do, the ability won’t trigger. If you don’t, the ability will check again as it tries to resolve. If you somehow control another colorless creature at that time, the ability won’t do anything. You won’t return Dust Stalker to its owner’s hand.
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.
Dust Stalker’s last ability doesn’t depend on Dust Stalker being colorless. If it gains one or more colors, the ability will still look for another creature you control 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.