Methods and kits for colouring hair are described. The method for colouring hair comprises (a) applying to the hair a first formulation comprising a coloring agent； and (b) applying to the hair a second formulation comprising an active agent, wherein the active agent contains at least one ionizable functional group and at least one functional group capable of forming a covalent bond with a thiol and having the formula: (B) m ---------Z-------- (A) n , wherein Z is a linker or is absent； B is a functional group capable of forming a covalent bond with a thiol and is independently selected from the group consisting of: A is an ionizable functional group which is independently selected from the group consisting of: -COOH, -SO3H, -P03H2, —N(R1)2, —N(R1)3； wherein step (a) occurs simultaneously with step (b).
