But for those that are still wondering?? I use Dupli-Color Engine Enamel with Ceramic. It is rated at 500* #DE1653 RED
It will burn off the bottom of most kettles and turn black, but it covers the rust. I have never had an issues with it on any lids.
You must realize it is just a cosmetic repair and it will eventually discolor under heavy use. But it is the best color match I have found off the shelf.
Go RED or stay home!