Adding any dry material, whether it is a colorant, a thickening agent, or a filler to epoxy will reduce the working time of the epoxy because it increases the volume of the epoxy without effecting the epoxy's chemistry. Epoxy generates more heat and reacts sooner when it is in a larger volume. Liquid additives (colorants, solvents) will slow the cure of epoxy, and can adjust the chemistry of the epoxy if too much is added.