Dye is a natural/synthetic substance that finds application for adding a color/altering existing color shade. As a coloring material, dyes are used to color commodities of day to day use and can be applied everywhere including * Plastic toys for children * Fabrics worn * From products like food * Wood items

Theory of Dyes Assignment Help
Theory of Dyes
A dye consists of a chromophore group and a salt forming group called anchoric group. In 1876, Otto witt put forth a theory as to correlate color with molecular structure (constitution). The theory is named'The Chromophore Auxochrome Theory' and its main postulates are,
(i) The color of the organic compounds is due to the presence of certain multiple bonded groups called chromophores. Important chromophores are, O || . . . . . . –N = N –, – N —> O, –N = N –, C = O, C = S Azo Nitro Carbonyl Thio carbonyl [Chromophore-Greek word, Chroma = color, Phorein = to bear].
The presence of chromophore is not necessarily sufficient for color. To make a substance colored, the chromophore has to be conjugated with an extensive system of alternate single and double bonds as exists in aromatic compounds.
The chromophore part of the colored substance (dye) absorbs some wavelengths from white light and reflects back the complementary color. A colored compound having a chromophore is known aschromogen.
(ii) Certain groups, while not producing color themselves, when present along with a chromophore in an organic substance, intensify the color. Such color assisting groups are called auxochromes(Greek word, Auxanien = to increase; Chrome = color), i.e. they make the color deep and fast and fix the dye to the fabric. The auxochromes are acidic or basic functional groups. The important auxochromes are,
Acidic : –OH, –SO3H, –COOH Hydroxy Sulphonic Carboxylic Basic : –NH2, –NHR, –NHR2 Amino Alkylamino Dialyliamino Example : HSO3 > N = N > N(CH3)2

However, Otto witt chromophore-Auxochrome concept fails to explain the color of certain dye stuffs like indigo.