Does devoid make a card's color identity colorless in Commander?
No, the reason that you cannot use Devoid cards in a Commander deck with a colorless commander is that a card's "color" is not the same thing as it's "color identity". "Color identity" dictates what cards that can go in your Commander deck, not "color".

Devoid changes a cards color.


  105.2. ...An object is the color or colors of the mana symbols in its mana cost...


Devoid does not affect the card's color identity.


  903.4. The Commander variant uses color identity to determine what cards can be in a deck with a certain commander. The color identity of a card is the color or colors of any mana symbols in that card’s mana cost or rules text, plus any colors defined by its characteristic-defining abilities (see rule 604.3) or color indicator (see rule 204).


If the rules text for Devoid said "This card's color identity is colorless" then things would be different.