The invention concerns the use as direct dyeing agent of at least one tetra-azapentamethine compound, a dyeing composition comprising at least such a compound, as well as a method for dyeing keratinous fibers, in particular human keratinous fibers and more particularly hair, which consists in applying at least one composition comprising such a compound. The composition also concerns tetra-azapentamethine compounds of formula (I). The invention further concerns a method for bleaching such compounds, as well as bleaching keratinous fibers previously dyed therewith.