What type of hair color is most damaging to the hair?

Permanent hair color is considered the most damaging to the hair due to the chemical processes involved in its application. This type of color often contains ammonia or other strong chemicals that open the hair cuticle, allowing the dye to penetrate the hair shaft for a long-lasting effect. The very nature of permanent color means it involves a more intense chemical reaction, which can lead to issues like dryness, brittleness, and breakage if not applied and maintained correctly.

In contrast, temporary color adheres only to the outer layer of the hair and typically washes out after a few shampoos, making it the least damaging option as it doesn't alter the hair's structure significantly. Semi-permanent color uses smaller dye molecules that penetrate the hair cuticle to a lesser extent than permanent color but still provides some lasting color. Natural color, of course, poses no risk of damage as it does not involve any artificial chemicals. Therefore, the structural changes and potential for damage from permanent color make it the correct answer.