Hmmm...that's a good question. NUPUR Henna is my lifeline..LOL!! Its the ONLY henna I use, and I've been using it close to 2years now
But I've never done an indigo treatment behind it, so I'd be interested in an answer too.
However, after monthly henna treatments for 2years, I do believe my hair has acutaully reached its henna "saturation". Its about as dark as its gonna get...I get no more color change anymore and I love it. My hair is now a permanent auburn-ish/burgundy-ish/darkbrown/near-black....and only available under the sunlight...LOL!!
Will have to look up NUPUR, not heard of it...
What I learned on hennasooq, it that the hennas make different shades of red...and then when you mix them with the indigo, it will cause the hair to turn dark brown or black...some of the hennas will cover gray better than others, so if you have grays to cover there is a different henna to use as your 'base'. Some hennas will make the hair have a 'redder' base..mmm dunno if redder is the word..but have a look at hennasooq dot com...i'll find the link and post it