hey, i've just recently (a week ago in fact) used the Colour B4. I've been colouring my hair for years, from blonde to black to a deep red, and was fed up of colouring over my roots, so i thought i'd try it. My hair is a natural dark blonde-ish colour, and pretty curly. And tbh, i was pretty pleased with the result i got! I used the extra strength stuff, as it recommends for lots of layers of colour. It does smell pretty bad when you use it (a bit like rotten eggs) and its a lengthy process. But the key is to keep rinsing and rinsing. I dont have ANY trace of colour left in my hair! It does make it pretty dry to begin with...But after you've finished the process and left to dry, just rewash with a lot of conditioner and it's perfectly fine!
I would say give it a try. Its not exactly breaking the budget compared to what you would have to pay in the salon!