Vat dyes are a type of dye that are insoluble in water and require a special process to apply them to fabrics. They are typically used on cotton and other cellulose fibers. The dye is first reduced to a soluble form, allowing it to penetrate the fabric. Once the dye is absorbed, it is oxidized back to its insoluble form, creating a long-lasting color. These dyes are known for their excellent lightfastness and washfastness, making them ideal for textiles that need to withstand frequent washing and exposure to sunlight. Common colors include deep blues and rich greens, often used in indigo dyeing.