Permanent haircolor most often works within which layer of the hair?

Permanent haircolor works mainly within the cortex of the hair. The product (typically with a developer) raises the cuticle so the color can penetrate into the cortex, where it develops and becomes trapped, producing long-lasting color. This is why permanent color is a chemical change and grows out rather than washing away quickly. Temporary and some semi-permanent colors deposit more on the surface/cuticle. Connecting where color works (the cortex) to the structure of the hair shaft is standard color theory on the exam.