Colour b4 on clean or dirty hair?

[blueemerald]

I've bought some colour b4 to use on my hair (dyed auburny coppery) because I want to try and get more of a strawberry blonde colour so need to take some of the darkness out.

My hair needs a wash (it's a bit greasy and has product in it), I generally wash it every other day and am "due" today but I can't be bothered to wash it and then go through the slightly arduous colour b4 process unless it will somehow make it less effective.

Does anyone know if the cleanliness of my hair will make a difference to how much colour is stripped out?