I haven't tried Amla, but I've read about it over the years. Maybe I'll give it a shot. Does it fade out, or does it make the henna bond to the strand in a different colour? I notice that henna never fades and is very permanent (which is great).
Mixing the henna and indigo and doing a one step process tends to fade to red pretty fast for me, and it's never quite dark enough. I did it for years but am a bit bored with it. I think I might go in a black direction next time, though it would be great if it was almost black but not quite. The indigo is still fading from my first attempt.
I've always used Yemen henna for the gray coverage. I'll have to try a few different ones.