If the red source you chose changes colors before it deals damage, the damage-prevention effect doesn’t apply.

If a permanent spell is chosen as the source of damage, Burrenton Forge-Tender’s prevention effect continues to apply for the rest of the turn to the permanent that spell becomes once it has entered the battlefield.

The last ability of Burrenton Forge-Tender doesn’t target anything. You choose a source of damage as the ability resolves, if able. You can activate it even if there is no source to choose.