The invention concerns the use of indoline derivatives of formula (I): in which R1, R2, R3, R4 and R5 independently of one another represent hydrogen or C1-4 alkyl groups, or R4 and R5 together with the oxygen atoms to which they are attached represent a C1-4 alkylenedioxy group, or of salts of such indoline derivatives, as additives to hair dyeing agents of the direct dyeing or primary intermediate/secondary intermediate type, thus producing hair dyeing with excellent color fastness characteristics.