What term best describes the process in which hair is subjected to a chemical in order to change its color?

The process of changing hair color through the application of chemicals is accurately described as color processing. This encompasses various techniques and methods used to alter the natural pigment of the hair, including the use of permanent, semi-permanent, and temporary hair colorents. During color processing, the hair's structure is manipulated through chemical reactions that either deposit or lift color in the hair strands, allowing for a variety of hues and shades.

While formulation refers to the specific mixture of ingredients used in hair coloring products, it does not capture the entire process involved in changing hair color. Direct dyeing pertains specifically to a method where color is applied directly to the hair without a developer, which is just one aspect of the broader process of color processing. A chemical reaction captures the scientific aspect of the change occurring at a molecular level but doesn't convey the procedural context of changing hair color in a salon environment. Thus, color processing is the most appropriate term to encompass the entire process of altering hair color through chemical means.