You can adjust the color of any wood putty, even bondo. In fact most woodputty comes in a natural color, it would be necessary to fix it to match walnut.
The colorants a paint store has in their machines to tint paint is a universal color tint. You might see if you can get them to dispense some burnt umber in a container for you. Add that to the famowood and it will match. If they don't have burnt umber you can mix raw umber and red oxide to make burnt umber. A little goes a long way so go easy with it. Probably less than a half teaspoon of burnt umber to a full can of famowood would do it. Be sure to try it out on scrap first.