I helped my friend dye the black cloth top on his 1925 Rolls Royce Twenty several years ago. I believe we used Renovo products. We spent several hours scrubbing it, and let it dry for a week, then applied the dye with foam paint brushes. It looked like a different car when done (If I could figure out how to post pictures here I could show a before and after). There was a run (on the side I did), but after some time in the sun it leveled out.

The best part was I got to drive the car some, but sadly this regal lady is back overseas as part of a large collection in France.