Permanently Darkening Vililigo Spots?

I have always had a patch (guess that's what you would call it) of vitiligo on my left arm. This means an area which has much less skin color. It's especially noticeable in the summer when I tan or burn. I use makeup to cover it, but would like to know if there is a way to permanently color it? I heard walnut oil may work to color, but is it permanent, and also, does anyone know what type of walnut oil? Any other ideas for permanent coverup?

Thanks for any help.