Acid dyes have complete chromatograms, bright colors, and the light fastness and wet fastness vary greatly with the variety of dyes. Mainly used for dyeing wool, silk and nylon, but also for leather, paper, ink, etc. Generally no coloring power to cellulose fibers.

Direct dyes have water-soluble groups such as sulfonic acid group (-SO3H) or cause group (-COON) arranged in a linear molecular structure. The aromatic ring structure is in the same plane, so the direct dye has a greater affinity for cellulose fibers, and it can be dyed directly in a neutral medium as long as the dye is dissolved in dry water.

Compared with the two, acid dyes have a simple structure and lack long conjugated double bonds and a flat structure, so they lack directness to cellulose fibers and cannot be used for dyeing cellulose fibers.