The present invention relates to a method to achieve a reversible change of the colour of hair dyed with an azo dye wherein the change of the colour can be effected by, for example, the salonist or the consumer in a simple manner.The colour of the hair thus dyed may be changed simply by treating the dyed hair with an acid treatment composition at any time after the first dyeing treatment. If desired by the consumer, the colour may be returned to substantially the original post dye colour by treating the hair further with an alkaline treatment composition.
