What part of the hair must be penetrated for effective color absorption?

For effective color absorption in hair, it is essential to penetrate the cortex. The cortex is the middle layer of the hair shaft and is where the majority of hair color and pigment resides. This layer contains long chains of keratin and is responsible for the strength, elasticity, and color of the hair. When hair color is applied, it needs to reach the cortex to alter the natural color by depositing or interacting with the pigment present in this layer.

The cuticle, which is the outermost layer, serves primarily as a protective barrier and does not retain color on its own. While some color products might slightly penetrate the cuticle, true and lasting color change requires interaction with the cortex. The medulla, which is the innermost layer of the hair, is typically present only in thicker hair types and does not play a significant role in color absorption. The follicle, located below the surface of the scalp, is involved in hair growth but is not directly involved in color absorption during the coloring process. Therefore, the most crucial component for color absorption is the cortex.