The present invention relates to a cationic 4-aminoindole of general formula (I), addition salts thereof with an acid and solvates thereof: in which: R1 is a linear or branched, saturated C2-C20 alkyl radical, substituted and/or interrupted with a cationic radical. The present invention is also directed towards a process for synthesizing this cationic 4-aminoindole, to the compositions, the uses, the hair dyeing processes and the devices using this cationic 4-aminoindole.