The present disclosure relates to the dyeing of keratin materials using thiol/disulfide styryl tetrahydroquinolinium fluorescent dyes. Disclosed herein is a dye composition comprising a thiol/disulfide styryl tetrahydroquinolinium fluorescent dye and a dyeing process with, for instance, a lightening effect on keratin materials such as hair, using said composition. Disclosed herein are thiol fluorescent dyes and the uses thereof in lightening keratin materials. This composition can be used to obtain a lightening effect which can be resistant and visible on dark keratin fibers.