A dye can generally be described as a coloured substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fibre.
Both dyes and pigments appear to be coloured because they absorb some wavelengths of light preferentially. In contrast with a dye, a pigment generally is insoluble, and has no affinity for the substrate. Some dyes can be precipitated with an inert salt to produce a lake pigment, and based on the salt used they could be aluminium lake, calcium lake or barium lake pigments.
Synthetic dyes quickly replaced the traditional natural dyes. They cost less, they offered a vast range of new colours, and they imparted better properties upon the dyed materials. Dyes are now classified according to how they are used in the dyeing process.
Dyes are generally defined along the lines of being coloured, aromatic compounds that can ionise. Although this definition infers that ionic interaction with oppositely charged tissue constituents is the norm, there are exceptions. Some dyes require the presence of a metal to properly develop their colour or staining selectivity. These are termed mordant dyes. The Colour Index uses this as a classification and naming system. Each dye is named according to the pattern: mordant + base colour + number.