I'm a believer in using a cover. To me the faded black kettles are evidence of sun damage but I have no proof of that.
I have an old redhead that has been outside, uncovered for years. It now has a cracking in the finish (see below). It could be related to high-heat cooks before I got the kettle, the bowl has some crazing, but I don't recall the kettle ever having that issue when I first got it.