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Colorants are chemical compounds that not only impart color to plastic materials but can also affect other properties, such as weather resistance, light stability, and transparency of the plastic. Colorants fall into two distinct classes: dyes and pigments; main distinction being dyes are soluble in the plastic matrix while pigments are insoluble. Most commonly used are the azo dyes that make up >50% of all dyes listed in the Color Index (Ambrogi et al. 2017) and used in textiles, paper, leather, rubber, or even foodstuffs (Ambrogi et al. 2017). Since organic dyes dissolve in the polymer, they do not scatter but only absorb light. Therefore, even at high concentrations of the dye, the plastic tends to be transparent or translucent. Some dyes such as aromatic amines, are known carcinogens and phthalocyanines have detectable estrogenic activity (Yang et al. 2011).

Pigments remain discrete particles that are well dispersed in the polymer matrix (Bolgar et al. 2016). Scattering and absorption of light by the pigment particles makes the plastic partly opaque (Andrady and Rajapakse 2019). Pigments are classified as either organic or inorganic. Organic pigments include benzimidazoles, quinacridones, and mono‐azos and provide the most brilliant opaque colors available (Ambrogi et al. 2017). Inorganic pigments are based on metals and can be divided into three classes: white pigments (TiO₂), carbon black (CB), and special effect pigment (Huckle and Lalor 1955). TiO₂ is the most widely used pigment in the plastic industry due to its high refractive index and ability to provide a high degree of opacity and whiteness. In addition, TiO₂ is known for its excellent durability and general nontoxicity. Black pigmentation in plastics is typically based on CB, the second most used pigments in volume by the plastic industry. CB also has dual functionality in that it can also act as a reinforcing filler, conductive filler, and light stabilizer improving the weatherability and stability of the plastic (Huang 2002). Special pigments impart vibrant colors to plastic materials and include fluorescent pigments, pearlescent pigments (mica coated with TiO₂), and metallic pigments (aluminum bronzes, copper, copper‐zinc alloys, and zinc).