In case anyone is interested, my experiment with Colourb4 has been amazingly successful. Having decided that as the front of my hair is totally white I was going to go natural, and faced with a dyed colour of mid brown I thought there was nothing to lose.
Two applications later, my hair is either white or blond, and the condition isn't bad at all.
Worth a try if you've had a dye disaster or want to lighten up imo