As previously established, color is a complex phenomenon, and to provide an absolute definition of pigment is not an easy task. Some definitions are provided below:

• Pigments are compounds that absorb light in the wavelength range of the visible region. This absorption is due to a molecule-specific structure (chromophore) that captures the energy from a radiant source. Some energy is not absorbed and is reflected and/or refracted; this energy is captured by the eye and generates neural impulses, which are transmitted to the brain, where they could be interpreted as a color.1

• The Dry Color Manufacturers Association makes a clear distinction between pigment and dyes: pigment is a colored, black, white, or fluorescent particulate organic or inorganic solid, which is usually insoluble and, essentially, physically and chemically unaffected by the vehicle or substrate into which it is incorporated. Thus, the pigmentation effect is by selective absorption and/or by scattering of light; a pigment will retain its crystalline or particulate structure. By contrast, dyes are soluble in the carrying medium and therefore crystalline/particulate features are lost in solution when a dyestuff is used to impart color to a material.2

In the latter definition, the difference between pigment and dye is emphasized. However, other authors prefer to use the more generic term colorant. Colorants are defined as substances that modify the perceived color of objects, or impart color to otherwise colorless objects. With this definition, pigments and dyes are grouped within the term colorants. It is reasoned that if only solubility is considered, the same substance could be a dye or a pigment depending on how it is used.3 It is important to be aware of such differences, but in our discussions we will use the terms colorants and pigments as synonymous.

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