For no doubt centuries walnut husks have been used as a natural dye yielding a walnut brown colouration to wood. For years we have sold a likely comparable product known as Van Dyke Crystals now difficult to source. This product is new to us in this form so some uncertainty until we get some feedback. It may be more a mordant which might require soaking the peels and straining the resulting liquid. 

If you use water as a solvent then realise that as with any water based dye some raising of the wood grain can be expected. This can be knocked back with ultra fine steel wool or very blunt abrasive paper.  

Soluble either in water (hot) or alcohol or some other solvents. Give a variety of dying colours, usable for different applications in violin making, such as antiquing or restorations. Cut peels in a plastic bag.