Indigo traditionally gives classic blue jeans their characteristic blue color.
Indigo is a deep blue dye used for centuries in textiles. In traditional dyeing, the dye vat contains a soluble form of indigo that penetrates the yarn. Exposure to oxygen then turns the material blue as the indigo returns to its insoluble form on the fibers.
Classic denim usually places indigo on the warp yarns while the weft remains undyed or lightly colored. As jeans are worn and washed, the outer indigo fibers gradually abrade and fade, revealing lighter shades. This process creates the worn patterns associated with vintage denim.
Modern jeans may use synthetic indigo rather than dye derived from plants. The color therefore does not prove that a pair used natural indigo. Indigo was important historically because it produced a strong, recognizable blue and could create varied fading as work trousers aged.