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Explanation of colorants from SPIE Press

The word colorant is a general term describing any substance used to modify the color of an object by changing its spectral transmittance, or its spectral reflectance. Colorants are either dyes or pigments. Technically speaking, the difference is that dyes are soluble in the host material—typically water—while pigments are not. Another difference is that dyes do not scatter light and look transparent. On the other hand, pigments do scatter light and, thus, they are opaque (see Fig. 6.12). Still another difference between dyes and pigments is that dyes are absorbed by the colored substrate and pigments need a binding agent in order to adhere to the surface. With a suitable chemical treatment, a soluble dye can be converted into an insoluble pigment. Unfortunately, the differences between a dye and a pigment are not commonly taken into consideration, frequently causing confusion. The definitions given in the dictionaries or used in some industries do not necessarily coincide with the technical terminology.