First; Most belt sandings are to coarse but dust from a orbital sander using 120 or finer paper will work if it is clean. Walnut powder turns black when wetted out by the epoxy. Use a light colored non resinous hardwood powder,like white hickory then add a little medium brown like oak or pecan or apple heart.when you get the color and consistency that you want it will harden to the same appearance .
Second;food color will work for some things but most have Propylene glycol in them and water. adding that to epoxy can give an elastomeric quality to the cured solid and in time cause deterioration. when adding a diluent to epoxy allow the parts to react for a minute before adding the liquid.
