I've used very fine pine sawdust or even light walnut sapwood sawdust to color epoxy too. The trick is getting an epoxy color match that will match the walnut when the final finish is applied. You can mix a few different small batches and compare them to a hidden area of the stock like the forearm channel that has a coat of the final finish, before you commit and end up with something that sticks out like a sore thumb. A small piece of walnut which closely matches your stock is good for a trial run too. Good reason to keep a box of those walnut scraps.
