Demi permanent hair dye is almost like permanent hair dye, except that it does not contain ammonia as the alkaline agent. It uses other types of alkaline agents such as ethanolamine and sodium carbonate, which is safer, more gentle and less damaging on the hair. It is also commonly known as deposits only hair color, as it just adds color to the hair without lightening the original hair strands, as the alkaline agents used are less effective in removing the natural pigments of the hair. As a result, it cannot color hair to a lighter shade. Deposits only hair dye is much more effective at covering gray or white hair than semi permanent dye, but less effective than permanent dye.
Although the process of coloring is almost similar to permanent hair dyes, with both having to be applied together with a developer, the concentration of hydrogen peroxide in the application of the developer for demi-perms (as they are also called) is usually much lower than the permanent dye. This gives it a less uniform or less solid hair color across the whole head than a permanent dye, as there is no lightening or bleaching of the natural hair pigments. The final hair color shades is dependent on the original colors of each strand, and may result in color shade differences due to the texture, thickness and structure of each hair strand. The resulting overall effect is a softer and more elegant natural looking hues.
Demi permanent dye is more suitable for fine or damaged hair than permanent dye, as it does not strip the natural hair entirely of its pigments in the coloring process, which may result in split ends, frizzly and brittle hair. Typically, demi-perms fade out over about 25 - 30 times of shampoo washes, as the color are just add-on deposits only.
The advantage is that as it fades out, the colored hair will naturally blend it with the root regrowth. As such, hair and color maintenance is much easier and not as costly because the hair regrowth is less noticeable, and recoloring and touch-up maintenance do not need to be done so often.
However, the usual allergy skin patch test should still be done before the application of the hair dye, as the deposits only hair dye is still hair coloring that contains chemicals which may have the probability to cause an allergic reaction when in contact with sensitive skin, or individuals who may be allergic to some of the chemical ingredients.