Indigo powder is applied same way as henna, but indigo itself does not really work on gray hair. It has to be either mixed with henna or applied on henna dyed hair. Here is a general idea on working with indigo powder to obtain desired results, these recommendations are assuming your original hair color is lighter than your desired color.

Desired Color Mixture
Light Brown 80% Henna + 20% Indigo +10% Cassia Obovata
Medium Brown 60% Henna + 25% Indigo+ 15% Cassia obovata
Dark Brown 40% Henna + 40% Indigo+ 20% Amla 
Black / JET Black 50% Henna + 50% Indigo (Two Step Process)


** Please note our pre-mixed colors i.e medium brown, dark brown contain other natural ingredients in correct ratios to obtain perfect desired results.