Bismuth Mindrender

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

If you cast a spell for another cost "rather than paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the spell has any mandatory additional costs, those must be paid to cast it.

If you choose not to cast the card, it remains in exile.

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.

You choose whether or not to cast the exiled card as Bismuth Mindrender's triggered ability resolves. If you do, you do so as part of the resolution of that ability. You can't wait to cast it later in the turn. Timing restrictions based on the card's type are ignored.