Indigo is a leaf powder that has a natural blue dye, so do not use indigo dye directly on grey hair, unless you want bluish-green colour. However, when Indigo is mixed with Henna, you can get different shades ranging from mid-brown to black.
Indigo is a much less stable dye than henna and releases dye much more quickly than henna. Therefore the indigo paste should be mixed just before applying to the hair.
Sometimes Indigo is sold as black henna. If the product is a bright green powder then it is most probably natural Indigo powder. If the colour varies from brown to black, then it is definitely not pure Indigo powder and has additives.
100% Indigo powder* (Indigofera Tinctoria)*denotes certified organic