What term refers to substances that are used to color hair, whether naturally or artificially?

The term that refers to substances used to color hair, whether naturally or artificially, is "colorants." This term encompasses a broad range of materials designed to impart color to hair, including both synthetic dyes and natural substances derived from plants and minerals. Colorants work by either depositing color on the hair or altering the hair's natural pigment.

While the terms "toners," "pigments," and "oxidizers" are related to the hair coloring process, they do not represent the full scope of substances used for coloring hair. Toners are specifically used to adjust or enhance the tone of hair after the initial color application, often refining unwanted undertones. Pigments refer to the actual coloring materials involved in dyes but do not cover the entire concept of hair coloring substances, which also includes various formulations and products. Oxidizers, on the other hand, are chemicals that facilitate the hair coloring process by opening the hair's cuticle or allowing the color to develop and typically accompany the coloring agents in many hair dye systems.

Thus, "colorants" is the most comprehensive term that fits the description of substances used for hair coloring in both natural and artificial contexts.