Ok, so I have a friend who has level 1 black and level 10 blonde vertical all through her head,(her natural color is a blonde,not bleach blonde like she has now) and now she wants the black to be brown. Can I use color oops on her whole head? Or will it turn out a disaster?

I used Colour B4 on my jet black hair and it came out, but the thing with bleached hair is that it's a colour stripper, so once you use bleach on your hair, dye over it a dark and then use colour remover it will always go back to the bleached stage because this is now the colour of the natural hair unless all the bleach has grown out, if you use black over bleach and then a colour remover you might get a stained bleach colour as the black is so intense it won't all come out, so my advice would be to use the colour oops,or colour before (which is quite good) and then a semi permanent darkish brown so its even all over. It has to be semi colour after using colour before, read instructions carefully.