The invention relates to a dyeing composition comprising a pyridinium thiol/disulphide fluorescent colorant having an external cationic charge and an interrupted alkylene chain, and to a method for colouring keratin materials, especially keratin fibres, particularly human keratin fibres such as hair, with a lightening effect, using said composition. The invention also relates to novel thiol/disulphide fluorescent colorants and the uses thereof for lightening keratin materials. Said composition produces a lightening effect which is especially tenacious and visible on dark keratin fibres.
