1. Permanent Hair Dye:
* Oxidizes Existing Melanin: Permanent hair dye contains ammonia or a similar alkaline agent, which opens the hair cuticle (the outer layer). It then uses hydrogen peroxide (or a similar developer) to oxidize the existing melanin pigments in the hair shaft. This process breaks down the melanin molecules, lightening their color. The extent of lightening depends on the concentration of peroxide and the duration of application.
* Deposits New Color: Simultaneously, the dye precursors (small, colorless molecules) penetrate the hair shaft. The peroxide causes these precursors to react with each other, forming larger, colored molecules within the hair. These newly formed dye molecules are trapped inside the hair shaft and create the desired new hair color.
2. Semi-Permanent Hair Dye:
* Deposits Color on the Surface: Semi-permanent dyes don't contain ammonia or peroxide. Instead, they deposit color molecules on the surface of the hair shaft and slightly penetrate the outer cuticle layer. They don't significantly affect the existing melanin.
* Melanin Still Visible: The natural melanin is still visible through the deposited color, which is why semi-permanent dyes are best for adding tone or depth to your existing color rather than dramatically lightening it.
* Washes Out: Since the dye molecules are not permanently trapped within the hair shaft, semi-permanent dyes gradually wash out with each shampoo.
3. Demi-Permanent Hair Dye:
* Low Volume Peroxide: Demi-permanent hair color contains a low-volume developer (peroxide) which allows some penetration into the hair shaft without drastically changing the natural pigment.
* Less Lightening, More Tone: Demi-permanent dyes deposit color, but they can also subtly lighten the hair by oxidizing some melanin. However, the lightening effect is much less pronounced than with permanent dyes. Demi-permanent dyes are good for blending gray hair, adding richness to natural color, or toning highlights.
4. Bleach:
* Breaks Down Melanin: Bleach contains a high concentration of hydrogen peroxide (or a similar bleaching agent) and an alkaline agent. Its primary purpose is to remove color from the hair by oxidizing and breaking down the melanin pigments.
* Significant Lightening: Bleach can lighten hair significantly, often to a pale yellow or white, depending on the original hair color and the strength of the bleach.
* Irreversible Damage: Bleaching can be damaging to the hair, as it weakens the protein structure.
In summary:
* Permanent dyes: Break down existing melanin to lighten hair and deposit new color.
* Semi-permanent dyes: Do not significantly affect melanin; they coat the hair with color.
* Demi-permanent dyes: Slightly affect melanin, depositing color with minimal lightening.
* Bleach: Drastically breaks down melanin for significant lightening.
The effect on melanin directly determines the level of color change achieved when dyeing hair. Understanding how each type of dye interacts with melanin is crucial for achieving desired results and minimizing damage.