The invention relates to an aqueous dye solution comprising a) 5 to 30% by weight of an anionic or cationic direct dye, b) 0.01 to 5% by weight of the compound of formula (1), in which M represents hydrogen, an alkali metal or alkaline earth metal, ammonium or ammonium, that is mono-, di-, tri- or tetra-substituted by C1-C4alkyl or C2-C4hydroxyalkyl or mixtures thereof, c) 0 to 10% by weight of an inorganic or an organic acid, d) 0 to 20% by weight of further additives and e) water to complete to 100%, with the proviso that direct dyes of the formula (2) in which K is a residue of a coupling component of the acetoacetanilide, pyridone, pyrazolone or pyrimidine series and M is as defined above, are excluded, a process for their preparation and the use thereof for dyeing paper.