A water-soluble formazan colorant, which is represented by the following general formula (I) in the form of free acid: ##STR1## (where: R represents a hydrogen atom or a lower alkyl group which may be substituted; X denotes a phenylene group or a naphthylene group which may be substituted; Y indicates --SO.sub.2 CH.dbd.CH.sub.2 group or --SO.sub.2 C.sub.2 H.sub.4 W group wherein W represents a group to be eliminated by the action of alkali; and Z designates a lower alkylamino group, a phenylamino group or a naphthylamino group, having one or two water-soluble groups selected from the group consisting of hydroxyl group, sulfonic acid group and carboxylic acid group); and a method of dyeing by use of the colorant. The colorant is used to dye cellulosic and nitrogen-containing fibers in blue shades with excellent reactive fixing properties.