Final answer:
Hair coloring products that gradually change hair color due to exposure to air and buildup are known as progressive dyes, not demi-permanent colors. Demi-permanent colors can last up to six weeks, offering a more natural look by considering the hair's original color and porosity, but are not effective for covering a lot of gray or white hair.
Step-by-step explanation:
Hair coloring products that change color gradually by progressive buildup and exposure to air are not typically demi-permanent colors; instead, they are referred to as progressive or gradual hair dyes. Demi-permanent hair dyes, indeed, wash out over time, typically within four to six weeks, and are less noticeable when the roots regrow. The color change is subtle because it is affected by the hair's original color and porosity. This makes the color appear more natural and less harsh compared to permanent dyes. However, semi-permanent dyes do not sufficiently cover gray and white hairs, which may not absorb the color at all. When gray and white hairs are widespread, a combination of semi-permanent base color with permanent highlights can be an effective approach to hair coloring.
In the context of permanent hair coloring, the process typically involves an oxidant and chemicals like diaminobenzene or diaminotoluene that undergo a complex chemical reaction to form larger dye molecules that are trapped within the hair shaft, giving a lasting color. Unlike semi-permanent and demi-permanent dyes, permanent hair dyes chemically alter the hair's structure, although the synthetic color will fade over time too.