Final answer:
Semi-permanent hair colors that include aniline derivatives require a careful application process similar to permanent dyes, involving the oxidation of the aniline derivative to achieve the desired color that lasts typically four to six weeks.
Step-by-step explanation:
Semi-permanent hair colors involve the use of less aggressive chemicals compared to permanent hair dyes. These products don't typically lighten hair because that process requires stronger chemicals like bleach. However, when semi-permanent hair dyes contain an aniline derivative, an additional step is necessary compared to temporary dyes or rinses that usually wash out after a shampoo.
Aniline derivatives are commonly used in the permanent hair coloring process. The first step in this process includes the oxidation of the aniline derivative, such as 1,4-diaminobenzene or 2,5-diaminotoluene, to a quinone state. Following this, it reacts with a coupler compound to form a larger dye molecule that gets locked inside the hair, thus creating the long-lasting color effect.
For semi-permanent options, the principle is similar but the molecules involved are designed so that the color washes out over a period, typically four to six weeks. Because of this, semi-permanent hair color is a good option for those seeking less commitment or fewer root touch-ups, which is especially useful for blending gray hair. In these cases, though the color fades, the initial application must be done correctly to achieve the desired shade, which often means that it requires the same precision and reaction conditions as permanent coloring, even though it doesn't involve lightening the hair.