A dye molecule is made up of two different part (i) Chromophore (ii) Auxochrome. The groups which produce colour in a compound i.e. , chromophores are usual unsaturated groups like nitro, nitroso, azo, azoxy, carbonyl and olefinic bonds Auxochrome are usually acidic / basic functional group like .

A compound containing only a chromophore may be coloured material but not a dye. For example azo benzene is red coloured but not a dye. Where as para amino azobenzene (aniline yellow) is a dye.