To which class of dyes does indigo belong?

Indigo belongs to the class of vat dyes, which are characterized by their near-insolubility in water and reliance on a reduction–oxidation process for fiber application. As one of the oldest and most widely used blue dyes, it possesses a highly stable conjugated chemical structure, endowing it with excellent color fastness to washing, light, and abrasion. Traditionally derived from indigo-bearing plants and later produced synthetically on an industrial scale, indigo is predominantly employed in the dyeing of denim and cotton textiles, giving them the distinctive deep blue hue widely recognized in apparel manufacturing.

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