I don't know, I had a friend in high school who was a fiber artist, worked a lot with natural dyes, and you just never knew what color her hands would be on any given day...

Maybe try a pumice scrub and just take off the stained outer layer(s) of skin???

Hopefully you won't need a surgeon the way I wound up needing for that thorn. (Just a local anaesthetic and a couple of little scalpel cuts, but too small for my PCP to deal with in her office.)