A compound of the formula ##STR84## or a salt thereof each cation of which is independently a non-chromophoric cation, wherein
- B.sub.b is ##STR85## wherein B.sub.1b is 1,4-phenylene;
- 1,4-phenylene monosubstituted by chloro, methyl, methoxy, sulfo or carboxy;
- ##STR86## wherein each R.sub.9b is independently hydrogen or sulfo, and
- Z.sub.2b is a direct bond, --NH--, --CH.dbd.CH-- or --CH.sub.2 CH.sub.2 --,
- R.sub.7a is hydrogen or methyl,
- R.sub.8a is hydrogen or methyl,
- Y.sub.2 is chloro, hydroxy, amino, 2-hydroxyethylamino, 2-sulfoethylamino, 2- or 3-hydroxypropylamino, di-(2-hydroxyethyl)amino, phenylamino, 3- or 4-sulfophenylamino or 3-carboxy-4-hydroxyphenylamino, and
- each Y.sub.3 is independently chloro, amino, 2-hydroxyethylamino, 2-sulfoethylamino, phenylamino or 3- or 4-sulfophenylamino,
- each D.sub.2 is ##STR87## or 1,5-, 3,6-, 4,8-, 5,7- or 6,8-disulfonaphthyl-2, wherein
- R.sub.2b is hydrogen, chloro, carboxy, methyl, methoxy, --COOR.sub.4b or acetamido, wherein R.sub.4b is methyl, ethyl or benzyl,
- R.sub.3b is hydrogen, chloro, methyl or methoxy, and
- m.sub.1 is 1 or 2,
- each M.sub.2 is ##STR88## or 1,4-naphthylene, wherein R.sub.5b is hydrogen, methyl, methoxy or sulfo, and
- R.sub.6b is hydrogen, methyl, methoxy or acetamido,
- each R.sub.1a is hydrogen or methyl, and
- each ##STR89## radical is in the 6-position of the naphthalene ring to which it is attached or each ##STR90## radical is in the 7-position of the naphthalene ring to which it is attached,
- with the proviso that the two D.sub.2 's are identical, the two M.sub.2 's are identical, and the two R.sub.1a 's are identical.