Wow, you poor guy. I'm sorry, I have no idea how to get walnut stains off your hands. I think I might try washing my hands with toothpaste. I doubt it will do much the first few times but it just might help if you're persistant.
I even asked Grandpa if he had any suggestions but he said what I said, "Wow". I then tried looking through some of my cleaning tip books but couldn't find anything. I'm hoping a visitor to Grandma's Wisdom might have a few tips to offer.