How do semi-permanent hair colors differ from permanent hair colors?

Semi-permanent hair colors are designed to deposit color onto the hair without penetrating the hair shaft as deeply as permanent hair colors do. This is a key distinction because it affects the longevity and intensity of the color.

Semi-permanent formulas typically sit on the outer layer of the hair, allowing for a temporary change in color that generally lasts for a limited number of washes. In contrast, permanent hair colors use a chemical process that opens the hair cuticle, enabling the color to penetrate more deeply into the hair fiber, which leads to a more lasting change.

Additionally, semi-permanent colors usually do not contain ammonia, which is a common ingredient in permanent formulas that helps facilitate deeper penetration and more lasting results. The fact that semi-permanent colors do not penetrate as deeply is what contributes to their gentler nature and the shorter time they last compared to permanent colors.