The chemistry of color: how permanent hair dye physically alters your hair
Changing your hair color is more than just applying paint. While temporary dyes sit on the surface, permanent color involves a complex chemical reaction that penetrates the hair's structure, breaking down natural pigments and trapping new, larger molecules deep inside the cortex.
To understand hair dye, one must first understand the hair's anatomy. Hair consists of three layers, the most relevant being the cortex and the protective outer cuticle. The cuticle is composed of overlapping scales that act as armor for the cortex, which houses the melanin proteins responsible for your natural color. The process of dyeing depends entirely on how deeply the pigment can penetrate these layers.
Temporary dyes function much like fine-grained paint, simply coating the cuticle scales and washing away easily. Semi-permanent dyes use much smaller pigment molecules that can slip between the cuticle scales to reach the cortex. However, because these molecules do not chemically react with the hair, they eventually wash out through the scales, typically lasting for a maximum of about 12 shampoos.
Permanent dye, however, is designed to last until the hair grows out or falls out. This process requires two components: an alkaline solution (such as ammonia) and a developer, which is usually a weak solution of hydrogen peroxide. The alkaline chemical works to soften and open the cuticle scales, providing a pathway to the cortex. Once inside, the developer acts as an oxidizer.
The developer performs two critical tasks. First, it oxidizes existing melanin, breaking the double carbon-carbon electron bonds within the protein. This turns the melanin colorless and releases sulfur atoms—a process that contributes to the characteristic odor of the dye. Second, the developer oxidizes colorless dye precursors. These precursors react with dye couplers to form large pigment polymers. Because these new pigment particles are too large to slip back through the cuticle scales, the color remains trapped beneath the surface.
Source: How Does Hair Dye Work?
