Personally I'd try an amla/henna mixture first. Indigo typically has some problems. It can lend a bluish cast to some people's hair, and reportedly "washes out" over time (more likely fades sometimes I think). Can't suggest ratios but there are lots of suggestions out there from reliable sources. Greys are harder to predict than some other hair colors as not only the COLOR of your hair has changed but also its texture/permeability.