Make henna up as directed and leave for dye release. When you are ready mix indigo and add together using the correct proportions, so 70% indigo and 30% henna will give you dark brown (66% indigo 33% henna also works - the more percentage of henna to indigo the lighter the colour towards light browns and the more percentage of indigo to henna the darker the colour towards dark brown). To get an almost nearly black you can mix 90% indigo to 10% henna this works if you have no grey hair but to get grey hair black, it is a two-way process, you will need to put henna on first, wash out and then apply indigo to hair.