Demi-permanent hair color is the "middle" type of hair color; not as long lasting as permanent hair color, but better able to cover grays than semi-permanent. Unlike semi-permanent hair color, it is made with an alkaline agent as well as a developer such as hydrogen peroxide, but in a much lower concentration than permanent hair color. This means that hair is not actually lightened when it is colored with this type of dye.
The obvious advantage to using demi-permanent hair color is that hair is not stripped of its color. The stripping process is very damaging to hair, causing it to become brittle and dry, particularly with repeated hair coloring. When using demi-permanent color, however, this is no longer an issue; the color is simply deposited on top of the hair. It covers gray hair much better than semi-permanent hair color, however, which is a main advantage.
In addition, grow out is less noticeable when using demi-permanent hair color. In general, the color will gradually wash out in between five and seven weeks, which is early enough that roots generally do not show. In addition, this type of hair color often looks more natural than permanent hair color, because it allows the hair to retain more of its natural color and color variance; hair that is all one shade tends to look unnatural.
In addition, demi-permanent hair color is easy to apply at home, and is sold in most drugstores, grocery stores, and discount stores. It is inexpensive and available in a wide range of shades, and may be used to create highlights as well. It is generally a better idea to use this type of hair color to darken hair; it will typically not work as well to lighten hair for the reasons mentioned above. Adding a bit of brightness or color dimension to hair with demi-permanent color is entirely possible, however.
Finally, demi-permanent hair color does not contain ammonia, which makes it gentler on the hair. This means that this type of hair dye can be used to refresh permanent hair color that has faded, or may be applied to the hair after a damaging treatment such as a perm or relaxing treatment. If looking for a hair color that is long lasting and good at covering grays, but not overly damaging to fragile, dry, or treated hair, demi-permanent color is often considered to be the best of both worlds.