What does Sulphur Bottom mean?
The process by which a sulphur dye is applied prior to indigo dyeing. As a result, a grey or yellow cast is achieved.
Heddels explains Sulphur Bottom
Sulphur bottom refers to jeans that have been treated with sulfur dyes before being dyed with indigo. This decreases the amount of time needed to achieve deeper colors though it does give the denim a gray or yellow cast.
Sulfur dyes are an economical choice to create darker tones in denim as it cuts down on the amount of indigo dye needed.