A liquid stripper comprising: - one or more strong acid(s), - a colored indicator capable of assuming at least three, and preferably at least four different colors depending on the acid concentration and on the polarity of the solution of stripper, the changes of color being observed in the range of pH of 0-3, notably of 0-2, and advantageously of 0-1, in addition to the fact that it is colorless for pH above 7, and - a surfactant, the colored indicator being in the form of a carbocation of the following formula in solution in the stripper: wherein A 1, A 2 and A 3 represent independently of each other, an aryl group, preferably phenyl, optionally substituted with one or several groups selected from a (C 1 -C 6)alkyl group, - OR 1, -NR 2 R 3, -SR 4, -CO 2 H and -SO 3 H, with: - R 1 and R 4 representing independently of each other, a hydrogen atom, a (C 1 -C 6)alkyl group, an aryl or aryl-(C 1 -C 6)alkyl group, the aryl rings being optionally substituted with one or more groups selected from -OH, (C 1 -C 6)alkoxy, (C 1 -C 6)thioalkoxy, (C 1 -C 6)alkyl, -NR 5 R 6, - CO 2 H or -SO 3 H, - R 2 and R 3 representing independently of each other, a hydrogen atom, a (C 1 -C 6)alkyl group, an aryl or aryl-(C 1 -C 6)alkyl group, the aryl rings being optionally substituted with one or more groups selected from -OH, (C 1 -C 6) alkoxy, (C 1 -C 6) thioalkoxy, (C 1 -C 6) alkyl, -NR 5 R 6, - CO 2 H or -SO 3 H, or R 2 and R 3 forming together a (C 4 -C 7)alkylene chain, and - R 5 and R 6 representing independently of each other, a hydrogen atom or a (C 1 -C 6)alkyl group, or R 5 and R 6 forming together a (C 4 -C 7)alkylene chain. - one or more strong acid(s), - a colored indicator capable of assuming at least three, and preferably at least four different colors depending on the acid concentration and on the polarity of the solution of stripper, the changes of color being observed in the range of pH of 0-3, notably of 0-2, and advantageously of 0-1, in addition to the fact that it is colorless for pH above 7, and - a surfactant, the colored indicator being in the form of a carbocation of the following formula in solution in the stripper: wherein A 1, A 2 and A 3 represent independently of each other, an aryl group, preferably phenyl, optionally substituted with one or several groups selected from a (C 1 -C 6)alkyl group, - OR 1, -NR 2 R 3, -SR 4, -CO 2 H and -SO 3 H, with: - R 1 and R 4 representing independently of each other, a hydrogen atom, a (C 1 -C 6)alkyl group, an aryl or aryl-(C 1 -C 6)alkyl group, the aryl rings being optionally substituted with one or more groups selected from -OH, (C 1 -C 6)alkoxy, (C 1 -C 6)thioalkoxy, (C 1 -C 6)alkyl, -NR 5 R 6, - CO 2 H or -SO 3 H, - R 2 and R 3 representing independently of each other, a hydrogen atom, a (C 1 -C 6)alkyl group, an aryl or aryl-(C 1 -C 6)alkyl group, the aryl rings being optionally substituted with one or more groups selected from -OH, (C 1 -C 6) alkoxy, (C 1 -C 6) thioalkoxy, (C 1 -C 6) alkyl, -NR 5 R 6, - CO 2 H or -SO 3 H, or R 2 and R 3 forming together a (C 4 -C 7)alkylene chain, and - R 5 and R 6 representing independently of each other, a hydrogen atom or a (C 1 -C 6)alkyl group, or R 5 and R 6 forming together a (C 4 -C 7)alkylene chain. - R 1 and R 4 representing independently of each other, a hydrogen atom, a (C 1 -C 6)alkyl group, an aryl or aryl-(C 1 -C 6)alkyl group, the aryl rings being optionally substituted with one or more groups selected from -OH, (C 1 -C 6)alkoxy, (C 1 -C 6)thioalkoxy, (C 1 -C 6)alkyl, -NR 5 R 6, - CO 2 H or -SO 3 H, - R 2 and R 3 representing independently of each other, a hydrogen atom, a (C 1 -C 6)alkyl group, an aryl or aryl-(C 1 -C 6)alkyl group, the aryl rings being optionally substituted with one or more groups selected from -OH, (C 1 -C 6) alkoxy, (C 1 -C 6) thioalkoxy, (C 1 -C 6) alkyl, -NR 5 R 6, - CO 2 H or -SO 3 H, or R 2 and R 3 forming together a (C 4 -C 7)alkylene chain, and - R 5 and R 6 representing independently of each other, a hydrogen atom or a (C 1 -C 6)alkyl group, or R 5 and R 6 forming together a (C 4 -C 7)alkylene chain.