I've seen adult appaloosa mules, so I'm thinking they don't typically fade out. If you find a tobiano (pinto from the mare not the jack), they typically stay, although they are minimally marked generally (often just white stockings).
I have seen a few spotted mules (from the jack, not the mare) that stayed and were colored similar to a spotted donkey, but I've seen more with the red like mine, and I think they do typically fade out.
I loved the red and wish it would have stayed, you could still see her markings very faintly, more so when she was wet. And the bottom of her mane and top of her tail were still darker. I sold her as a 3 year old so don't know how she changed after that.