Can I mix walnut hulls AND tag Alder together for a darker hair dye?

I am allergic to commercial hair dye. I have used walnut hulls for dye before but it is not dark enough to cover my grays in my dark brown hair. The grays are no longer gray but still stick out because they look lighter brown than the rest of my hair. Would combining several of these natural darkening remedies work to make the dye darker? Maybe combinations of sage, walnut and tag alder + coffee? Has anyone tried this? Would there be any reason not to? *because my hair is curly I am not interested in henna as it straightens curly hair-I like my curls.