Sulphur dyes (Sulfur dyes in the US) were at one time the most commonly used textile dyes. They are still the preferred dye type for cellulose based fabrics. They are available in a wide range of colours.

The first widely available sulphur dye was Leuco Sulfur Black 1, which was commercialised over 100 years ago.

In general, the exact chemical structure of the entire group is unknown.

Technical details :

Essentially they are dyes which contain aromatically attached oligo sulfur linkages which are reducible to thiol groups and which may be of oligomeric or polymeric structure. They are mainly obtained by thionation of the respective intermediates at high temperature, for example above 100°C, especially in the temperature range from 1 10 to 300°C (eg by baking or in the melt or in the presence of water and/or an inert organic solvent). The exact structure of sulfur dyes is generally unknown or incompletely known, even though the starting materials are known, since, for example, the number or position of the oligo sulfur linkages, the number of sulfur atoms in an oligo sulfur linkage, the number of hetero rings and/or NH linkages and/or the number of structural repeat units in the sulfur dye molecule may vary according to starting materials and synthetic conditions.

Source : European Patent EP1200669A1

For an overview, see : Wikipedia