One thing that will turn your walnut stock gray is Hydrochloric Acid. It isn't nearly as frightening as it sounds. They use it in swimming pools and hot tubs but call it Muriatic Acid.
This stuff turns pool deck wood gray very quickly. I haven't tried it on walnut because it doesn't appeal to me but I have seen it used to remove mold on everything from pine to oak and maple. The wood turns grey very quickly, as in less than a week if the acid is left on without washing it off right away. Usually when you scrub down a deck with it you want it to soak into the wood to inhibit mold spores from sprouting. So the stain goes deep.
Give it a try on a piece of scrap wood first to see what happens.