Be aware that the bigger the crack the less convincing a pigmented patch will look, because Walnut doesn't have large areas of completely uniform colour. Personally I work on the basis that if the void is greater than about 2 or 3mm pigment is best avoided, and if it's less than 2 or 3mm then IMO pigment isn't necessary!
I might use pigment on something like Cherry where resin pockets are common, so you can pass off a patch as a resin pocket, but on English or American Walnut I'd follow a different tack. For a dead knot I'd drill out the offending knot and find a smaller, acceptable live knot on a matching piece of scrap, then take that out using a matching plug cutter and tap it into the hole. Done correctly that really is an invisible patch. For a crack I'd either plug the void with a Walnut shim, or fill it and over coat with shellac before painting in the grain patterns with acrylic, then seal it with another coat of shellac before applying the final finish.
Good luck!