Final answer:
A developer, usually containing hydrogen peroxide, is the oxidizing agent used with demi-permanent and permanent hair colors, lighteners, and toners to open the hair cuticle and facilitate the dyeing process.
Step-by-step explanation:
The oxidizing agent used with demi-permanent and permanent colors, lighteners, and toners is known as a developer. The role of the developer is to open the hair cuticle so that the coloring agent can penetrate the hair shaft. It typically contains hydrogen peroxide, which is also used as a disinfectant and for bleaching.
This substance is crucial in the hair coloring process as it aids in the first step by oxidizing the dye precursor, allowing it to react with the coupler to form the dye, and in the final step to lock the dye in place, resulting in a permanent coloration effect on the hair.