A compound represented by formula (I):Formula (I) ##STR00001## wherein Z.sub.1 and Z.sub.2 each are atoms necessary for forming an aromatic ring; V.sub.1 and V.sub.2 each are a substituent W.sub.1 or W.sub.2; when at least one V.sub.1 is W.sub.1, at least one V.sub.2 is W.sub.2, or when at least one V.sub.1 is W.sub.2, at least one V.sub.2 is W.sub.1; r is 1 to 4; s is 1 to 4; M.sub.1 is a counter ion; m.sub.1 is the number necessary for neutralizing charge; W.sub.1 is a hydroxyl, primary- or secondary- or tertiary-amino, acylamino, or sulfonamido group; W.sub.2 is a nitro, cyano, alkoxycarbonyl, aryloxycarbonyl, alkyl- or aryl-sulfonyl, carbamoyl, sulfamoyl, alkenyl, alkynyl, aryl, heterocyclic, sulfo, carboxyl, heterocyclic oxy, ammonio, alkyl- or aryl-sulfinyl, alkyl- or aryl-sulfonyl, acyl, or aryl- or heterocyclic-azo group; and the aromatic ring may have a substituent other than V.sub.1 and V.sub.2.