Well.....I would suspect sun damage but that's just an opinion. In my example above the black and the blue ink on the inlay stayed intact while the other colors nearly completely faded along with the bronzing of the inlay. So sun damage seems to be a function of the inlay materials and the types of inks and will vary chip to chip.
I posted these two chips a few months back. I believe the brown chip was born identical to the red one but I'm still mystified. I've never seen a similar example. Whatever the process was it only and exactly affected the clay while the multi-colored inly is intact.