I thought I'd post my experience here now that I tried Colour B4.

I have applications of henna+cassia, pure henna, henna + indigo on my head, about two (?) of each.
My virgin roots are between chin and shoulder right now, the henndigo made the hair a nice brown without red undertones, the difference between virgin and dyed was hardly noticeable to anyone but me. Now why would I want to change that?

I wanted to go blonde or at least get some highlights, so I figured I needed to get rid of the henndigo first, so it would not turn green from the peroxide.

My DH helped me apply Colour B4 to the dyed parts of my hair only, I left it in for about three or four hours. Rinsed like crazy, with a couple of shampooings inbetween. Went to bed with wet hair. Woke up with virgin roots+moss green hair
The Colour B4 seems to have worked well enough, removing the Henna. The indigo however, did not budge, resulting in that lovely vegetable shade.

It sounds worse than it is. The noticeable green has settled down somewhat with the oxidation process and no one would call my hair green now, especially in sunlight, where a teeny bit of red comes through and neutralizes the green.

My hair was very dry and tangly the next couple of days, but a SMT, aloe vera gel and oiling have returned it to its natural state.

I would not try it again on my henndigo, but I will be switching to chemical dyes for a while now. I'd defintely use Colour B4 again to remove those.