If you leave the indigo on for longer you're likely to go black. When you do it's irreversible. It can still give a reddish hue in the sun but the colour would be black.
I used to do the 2-step method for years. Not for the colour (my hair is nearly black) but for the conditioning purposes. I love the effects of henna but with indigo I tried to cover the redness of henna. Now with the CG method I haven't found the henna necessary.
The indigo might fade of the gray strands but with repeated use it should stick better. I used to add salt to the indigo (1 tbs) and amla to my henna.
Be careful with indigo if you want to avoid black. I would recommend the mix of henna-indigo instead of 2-step if you want dark brown.
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