Amla cools down the red from henna. What I've noticed is that when used with henna in the 2-step process it makes indigo stick and hold better. Looks like the indigo fades less when used on henna-amla mix.
This is how I do my 2-step. I mix 100g of Moroccan BAQ henna (ie. Tazarine) with some lemon juice and warm water. I might add some crushed cloves just for the smell. I cover the mix with plastic and place the bowl in a warm place. After the colour release I mix in the amla, maybe about 50 g. I let the henna stay on my head for about 2 hrs, then rince. Right after I mix 100 g of indigo with a table spoon of salt, then I add warm water. I spread the indigo on my "towel dry" hair and cover my head with plastic as quickly as possible. Indigos dying power fades when in touch with oxygen. I leave the indigo on for 1 h, rince well and voilą, black hair that shouldn't fade.
Not sure if this helps but it might give some ideas.
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