To be honest, i wouldn't bother with Head & Shoulders, fairy etc, it won't lift the black out enough. Just get Colour B4 and apply that. As mentioned before, you must follow the instructions to the letter, especially the rinsing part.
You may end up with a gingery tone, if so (i would recommend you buy this at the same time as buying Colour B4 - at least you will have this if you should need it) you can apply this immediately after -
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this will subdue any brassy gingery tones - unless you like it a bit gingery! DON'T put a permanent colour on for at least 4 weeks, your hair will return to a dark shade again if you do. You can use a semi-permanent - but choose a colour at lease 2 shades lighter than what you want to be to avoid your hair going too dark again.