Temporary and semi-permanent hair colors change hair in an extremely limited way because they only coat the outside of the hair (the cuticle layers) with color. This color can be plant-based, like henna, or can be a synthetic coloring agent. Regardless of the source, these types of dyes do not penetrate past the cuticle and do not change the internal structure of the hair shaft in any way. As such, these types of dyes are good choices for the color-shy or those who want only an extremely subtle color change (and don’t have much, if any, gray hair to camouflage).

In order for demi, level 2, mid, and intermediate or permanent/level 3 hair dyes to work, they require what is often referred to as a double process. They all do this using the same basic ingredients, and cause the same remarkable chemical process to take place:

1. The first step involves using an alkaline base (almost always at a pH of 10 to 11) to swell the hair shaft, opening it up to make the cortex permeable so the precursor hair-dye molecules can penetrate and remain in the heart of the hair.
   
   
2. Hydrogen peroxide in some amount (the amount varies depending on how the product was formulated) lifts your existing color from the cortex.
   
   
3. Once the precursor dye molecules are in the cortex, they go through a chemical process that is turned on during the time it sits on your hair, forming large molecular chains of color that are now big enough to remain inside the hair-shaft and are less likely to wash out. It is the action of the peroxide that gives these demi and permanent hair-coloring products the name oxidative hair dyes.

On a microscopic level, the dye molecules in permanent hair colors are capable of a limitless variety of tones, hues, tints, and intensities. Cosmetics chemists with computer simulation programs have concocted thousands upon thousands of potential color combinations for the hair. These dye molecules are a matter of pride for each of the companies that make hair-coloring products. Whether the product is a cream, lotion, gel, or foam, the product's pH level (it must be alkaline), the molecular structure of the dye, and the volume (concentration) of peroxide are what determine the outcome. Regardless of the claims, no matter how natural, organic, or moisturizing the product says it is (which they aren't), demi and permanent hair dyes are damaging to the hair. However, they can create magnificent color changes that are simply to dye for (Source: Journal of Cosmetic Science, July–August 2003, pages 395–409).

Paula Begoun