Traditionally, colorants have been classified into dyes and pigments which are being used in the process of coloring materials. Coloration is a physical process whereby we apply dyes to textiles or incorporate pigments into paints, inks and plastics by dispersion.
But today, we can no longer limit colorants to conventional dyes and pigments.
Color means an effect perceived by an observer and determined by the interaction of the three components of light source, object and observer.
Chromatic colorants have selective absorption and scattering of light which vary with the wavelength. So they can selectively modify the spectral power distribution of light falling on them. Achromatic colorants such as black and grey are spectrally non-selective absorbers while white pigments are spectrally non-selective scatterers.
In short, for our understanding, we can say, colorant is a substance that is used to impart colour to objects. But at the same time, the general classification into dyes as soluble and pigments as insoluble, does not hold any ground in recent years.
If the colorant has an affinity for the substrate (textile, paper etc.) and will become a part of the colored material without the need of an intermediate binder, we consider such a colorant to be a dye. Dyes, whether, direct, vat, disperse, solvent-soluble or any other type, are dyes because a binder is not required to hold them on the material being colored.
On the other hand, the pigment does require a binder so that it is fixed to the substrate as it does not have affinity to the substrate. A pigment applied to a surface without a binder will not adhere to the surface.
Author: Sambhaji Chopdekar, Institute of Chemical Technology, Mumbai.
Source: Billmeyer and Saltzman’s PRINCIPLES OF COLOR TECHNOLOGY (Third Edition) by Roy S.Berns, A John Wiley and Sons Co., USA (2000)