Bisazo dyes and mixtures thereof

A compound of formula (I)

• wherein K1 and K2 are independently unsubstituted or substituted aryl, and
• wherein substituents of the substituted aryl are selected from the group consisting of H, —SO3H, unsubstituted or substituted, linear or branched C1-6 alkyl, unsubstituted or substituted, linear or branched C1-6 alkoxy or SO2Y, wherein Y is —OH, —CH═CH2 or —CH2CH2—Z, wherein Z is a leaving group which can be eliminated by OH− ions.