Towel method is working pretty well. You can see the black residue. It takes a couple of minutes. Warming the button caps up a bit helps. I washed them with warm water and a bit of dish soap afterwards.
Syn1 after rubbing and washing, almost done. The up arrow still needs some work. Syn2 hasn’t been worked on, yet. Syn1 looked like Syn2 before it got its towel treatment.
Edit, be careful not to rub the color off completely, that can also happen apparently. But honestly, I’d rather have button caps where the led shines through a bit on the edges than button caps that stick to my finger tips^^