Sulphur dyeing uses a family of dyes that, unlike indigo, are commonly used to achieve colors such as black, grey, dark green or brown in denim, in addition to being used on its own or blended with indigo for particular shades of blue. The dye is applied in a process broadly similar to indigo dyeing, forming a layer that sits largely on the surface of the yarn.
Because sulphur dyes also tend to sit near the surface of the fiber, sulphur dyed denim can fade with wear in a way that is visually similar to indigo fading, gradually revealing lighter cotton beneath as the surface color wears away. This makes it useful for creating black or colored jeans that still develop the worn in character associated with indigo denim.
You will most often meet sulphur dyeing on black or dark colored jeans, where pure indigo alone would not give the desired shade. See our jeans washes and finishes guide and the entry on indigo for how the two dyeing approaches compare.