An agent for coloring and lightening keratin fibers, in particular human hair, containing, based on its weight, from 0.001 to 15 wt.% of at least one compound of formula (Ia): in which A, C represent, independently of one another, an optionally substituted, in particular halogenated, C 1-28 alkyl group, preferably an optionally substituted, in particular halogenated, C 1-6 alkyl group, or a C 2-28 alkylene group, preferably a C 2-6 alkylene group, wherein the functional groups A and C can together form a ring, and wherein preferred functional groups A and C are selected from -CH 3, -CH 2 CH 3, -(CH 2) 2 CH 2, -CH(CH 3) 2, -(CH 2) 3 CH 3, - CH(CH 3)CH 2 CH 3, -CH 2 CH(CH 3) 2, -C(CH 3) 3, -CF 3, -CCl 3, - CBr 3, -Cl 3, -CF 2 CF 3, -CCl 2 CCl 3 n represents 0, 1 or 2, preferably 0, from 0.5 to 15 wt.% of hydrogen peroxide (calculated as 100% H 2 O 2), additionally from 0.001 to 5 wt.% of one or more oxidation dye precursors and/or direct dyes, and from 1 to 20 wt.% of at least one imidazole compound according to formula and/or the physiologically acceptable salts thereof, where R 1 represents a hydrogen atom, an optionally substituted aryl group or a (C 1 -C 6) alkyl group, R 2 represents a hydrogen atom, a carboxaldehyde group, a (C 1 -C 6) alkyl group or a nitro group, R 3 represents a hydrogen atom, a carboxy (C 1 -C 6) alkyl group, an amino (C 1 -C 6) alkyl group, a carboxyl group, a carboxaldehyde group, a (C 1 -C 6) alkyl group, a nitro group, a 2-amino-3-hydroxypropyl group or a -CH 2 -CH(NH 2)-COOH group, R 4 represents a hydrogen atom, a carboxaldehyde group or a carboxyl group, wherein the imidazole compound according to formula and/or the physiologically acceptable salts thereof is/are selected from histamine, D-histidine, L-histidine, DL-histidine, D-histidinol, L-histidinol, DL-histidinol, imidazole, imidazole-4-acetic acid, imidazole-4-carboxylic acid, imidazole-4,5-dicarboxylic acid, imidazole-2-carboxaldehyde, imidazole-4-carboxaldehyde, imidazole-5-carboxaldehyde, 2-nitroimidazole, 4-nitroimidazole, 4-methylimidazole-5-carboxaldehyde, N-methylimidazole-2-carboxaldehyde, 4-methylimidazole, 2-methylimidazole, N-methylimidazole, N-(4-aminophenyl)-imidazole, and the physiologically acceptable salts of said compounds. A, C represent, independently of one another, an optionally substituted, in particular halogenated, C 1-28 alkyl group, preferably an optionally substituted, in particular halogenated, C 1-6 alkyl group, or a C 2-28 alkylene group, preferably a C 2-6 alkylene group, wherein the functional groups A and C can together form a ring, and wherein preferred functional groups A and C are selected from -CH 3, -CH 2 CH 3, -(CH 2) 2 CH 2, -CH(CH 3) 2, -(CH 2) 3 CH 3, - CH(CH 3)CH 2 CH 3, -CH 2 CH(CH 3) 2, -C(CH 3) 3, -CF 3, -CCl 3, - CBr 3, -Cl 3, -CF 2 CF 3, -CCl 2 CCl 3 n represents 0, 1 or 2, preferably 0, from 0.5 to 15 wt.% of hydrogen peroxide (calculated as 100% H 2 O 2), additionally from 0.001 to 5 wt.% of one or more oxidation dye precursors and/or direct dyes, and from 1 to 20 wt.% of at least one imidazole compound according to formula and/or the physiologically acceptable salts thereof, where R 1 represents a hydrogen atom, an optionally substituted aryl group or a (C 1 -C 6) alkyl group, R 2 represents a hydrogen atom, a carboxaldehyde group, a (C 1 -C 6) alkyl group or a nitro group, R 3 represents a hydrogen atom, a carboxy (C 1 -C 6) alkyl group, an amino (C 1 -C 6) alkyl group, a carboxyl group, a carboxaldehyde group, a (C 1 -C 6) alkyl group, a nitro group, a 2-amino-3-hydroxypropyl group or a -CH 2 -CH(NH 2)-COOH group, R 4 represents a hydrogen atom, a carboxaldehyde group or a carboxyl group, wherein the imidazole compound according to formula and/or the physiologically acceptable salts thereof is/are selected from histamine, D-histidine, L-histidine, DL-histidine, D-histidinol, L-histidinol, DL-histidinol, imidazole, imidazole-4-acetic acid, imidazole-4-carboxylic acid, imidazole-4,5-dicarboxylic acid, imidazole-2-carboxaldehyde, imidazole-4-carboxaldehyde, imidazole-5-carboxaldehyde, 2-nitroimidazole, 4-nitroimidazole, 4-methylimidazole-5-carboxaldehyde, N-methylimidazole-2-carboxaldehyde, 4-methylimidazole, 2-methylimidazole, N-methylimidazole, N-(4-aminophenyl)-imidazole, and the physiologically acceptable salts of said compounds. R 1 represents a hydrogen atom, an optionally substituted aryl group or a (C 1 -C 6) alkyl group, R 2 represents a hydrogen atom, a carboxaldehyde group, a (C 1 -C 6) alkyl group or a nitro group, R 3 represents a hydrogen atom, a carboxy (C 1 -C 6) alkyl group, an amino (C 1 -C 6) alkyl group, a carboxyl group, a carboxaldehyde group, a (C 1 -C 6) alkyl group, a nitro group, a 2-amino-3-hydroxypropyl group or a -CH 2 -CH(NH 2)-COOH group, R 4 represents a hydrogen atom, a carboxaldehyde group or a carboxyl group,