What ingredient in hair color acts as a developer, allowing color to penetrate the hair?

Hydrogen peroxide serves as the developer in hair color formulations, playing a critical role in the coloring process. When mixed with the hair dye, hydrogen peroxide triggers a chemical reaction that opens the hair cuticle, enabling the color molecules to penetrate the hair shaft effectively. This penetration is essential for achieving the desired hair color and ensures that the color adheres to the hair.

Ammonia is often included in hair color, but its primary function is to raise the pH level of the hair, which helps open the cuticle, rather than acting as a developer itself. Conditioner is not involved in the coloring process; instead, it is used to smooth and protect the hair after coloring. Color pigment refers to the actual dye components that provide the hue but does not assist in penetrating the hair. Thus, hydrogen peroxide is the correct answer as it directly facilitates the development and application of hair color.