if the brown colour u already have is going to be used to colour the white part back to brown, this colour would have to be very warm other wise it is likely to fade very fast and be left with a khaki green shade. if the shade is not very warm i would advise that it be pre pigmented before you re colour it. This would require the blonde to be coloured red/copper (depending on the depth of the brown) to re pigment the blonde. once this has been done you can colour the blonde with the dye. . can i ask are you doing this urself or asking because u want to know what to ask for when u go to a salon. . if it is in a salon they will know all this already, if u are doing it urself, tread carefully as often recolouring blonde can go quite wrong and the colour will not be as desired. . x
