Vat dye

The most common reducing agent is sodium dithionite (Na2S2O4), which converts the dye to its "leuco" form that is soluble.

Once attached to the fabric, the leuco dye is then oxidized to the insoluble state which is intensely colored.

Indigo is an example of this dye class: it changes from yellow, in the dyebath, to green and then blue as the air hits it.

On the other hand, vat dyes tend to have poor rubbing fastness, but this can be mitigated with special treatments to the fabric.

Indigo is subject to major crocking (i.e., rubbing the dye off onto other items) unless it is applied carefully.