I don't have any medical training and don't know much about this. Afaik, lupus can cause problems in the nails - possibly discolouration. Vitiligo definitely seems to cause discolouration in the nails and possibly other problems, as well. It's a bit like the red 'butterfly' rash on his cheeks caused by the lupus, a physical manifestation of a serious disease. An auto-immune disease can cause problems which are visible (the nails) and other problems which are not visible (Michael feeling very cold all the time, that would be the lupus affecting his core temperature).
I just love the fact that his hands were beautiful, expressive, sensitive-looking and just really, really interesting. I love the way his hands look but also the way he uses them.