As stated, bleaching will lighten the color. BUT, once done, your stuck with it. As walnut is exposed to sunlight, it lightens on its own. If you ever look at more modern firearms that may have one side lighter then the other... that is the side that was exposed the most, maybe hanging on a rack on a wall. Some folks would lighten walnut and then stain so as to get more of an orange color, seen that done on a few Frank Lloyd Wright (Wrong by those of us in the know) projects.
If your matching to older stock parts, it's a tough experimental thing. Best results will require wood with matching pore sizes and grain.