Indigo is an example of

These are water insoluble coloured compounds. They can be reduced to colourless (leuco) compounds, that are soluble in alkali and are easily reoxidized to give the dye. These dyes dye both animal and vegetable fibres directly. Mostly they are used for cotton fibres. The cloth treated with alkali is oxidised by air which makes the dye return to the insoluble form. For example: Indigo