Christopher had a formula for blondes and non-blondes. The non-blond formula uses darker herbs and I think he just didn't want blondes to have stained hair; his method was to wash your hair in the herbs and the blondes used herb that were blonde in color to begin with.
I never use the topknot on anything but the temples, so I know it is due to circulation and nothing to do with the color of the solution.