Patent ID: 7198651

Claim:
A method of effecting a change of colour of hair, comprising the following steps: (a) dyeing the hair to a first colour with an aqueous alkaline dyeing composition comprising at least one azo dye of formula (I): A-N═N—B (I) wherein A is a monovalent group, which binds an azo group with a carbon atom, and which is free of carboxy (—CO 2 H) groups, sulfo groups (SO 3 H), or quaternary ammonium groups, and B is a monovalent group containing a dissociative proton and, which is free of carboxy (—CO 2 H) groups, sulfo groups (—SO 3 H), or quaternary ammonium groups, wherein the azo dye is capable of reversible colour change of hair in an aqueous medium; (b) rinsing the dyed hair with water and (c) subsequently treating the dyed hair with an aqueous acid formulation not containing substantially any dye, wherein the colour of the hair is changed to a second colour.