I have no idea what it would look like on walnut, but have achieved some beatuiful results on Curly Maple with the old mission furniture technique of fuming with strong ammonia. This was originally applied on Oak which contained tannic acid. For other woods you simply brew a strong tea solution & use for raising the grain. After this place in a covered container & set an open dish/bowl/whatever of strong ammonia in with it. leave overnight or until it takes on the desired color.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra