Sulphur Dye

Dyestuff derived from a sulphur base used alone or in combination with indigo to change the cast of the shade. Using sulphur dye, indigo can be made a variety of blue casts from green and yellow to black and brown. The sulphur dye can be appiied to yarns before the indigo (sulphur bottom) or after the indigo (sulphur top).

Source: Tellason Denim Terminology