Acid dyes refer to dye molecules containing acidic groups, also known as anionic dyes, which can combine with amino groups in protein fiber molecules through ionic bonds and are suitable under acidic, weakly acidic or neutral conditions. Mainly azo and anthraquinone structures, a few are aromatic methane structures. Dyes and colors are generally compounds that have their own color and can make other substances obtain bright and firm colors in a molecular or dispersed state.

Acid dyes have a complete chromatogram and bright colors. The light fastness and wet processing fastness vary greatly with different dye varieties. Compared with direct dyes, acid dyes have a simple structure and lack long conjugated double bonds and a co-planar structure. Therefore, they lack directness to cellulose fibers and cannot be used for dyeing cellulose fibers. Different types of acid dyes have different dyeing properties due to different molecular structures, and the dyeing methods used are also different.