Coffee does work, but you need to use fresh not dregs, boil it STRONG and the best is French roast. Also it is not a wipe on dye - you must soak the piece in it for up to several hous much like you do with walnut.
Mark - Walnut can and will grow mold, but two ways to prevent that: add some alcohol or freeze it in smaller containers and use as needed.
Another option besides those noted, for those wanting to use brown or chestnut is to buy it pre-dyed from Wickett & Craing who will also split the leather to what ever thickness you like for free.