It all comes down to a little something I haven’t thought much about since high school: chemistry. “There are many different chemistries that can be used to color hair, and the categories have some overlap,” explains cosmetic chemist Kelly Dobos. “Permanent dyes involve the penetration of small color molecules into the hair, which are then converted into larger molecules through a chemical reaction. Those larger molecules become embedded in the hair.” In other words, they won’t wash out.
Semipermanent dyes, on the other hand, “rely primarily on diffusion of dye into the hair cuticle,” says Dobos. “No chemical reactions involved.” The dye molecules aren’t embedded in the hair, which means a semipermanent dye will continue to wash out with each shampoo.
Demipermanent dyes fall somewhere in between. The magic (er, chemistry) is in the developer. “Developers help open the hair cuticle, allowing dye molecules to penetrate,” says Dobos. Depending on the level of developer you use, it can also provide some “lift” or lightening of the hair. “The degree of lift is dependent on the concentration of peroxide in the developer,” she continues. “The higher the volume number, the greater the lift.” But a low-level developer, like a 10 volume, “allows pigment to deposit with no lift.”
Okay, chemistry lesson over. Demipermanent dye uses a low-volume, mild peroxide developer (usually a 10 volume developer) to enable color molecules to penetrate and coat the surface layer of the hair cuticle — and just the surface layer. Because of the low concentration of peroxide, it won’t lighten your hair (good news, it won’t damage it either). So it’s not a great choice if you’re hoping to go blonde, but it is a great option for blending uneven color, adding lowlights, toning dull or brassy blondes, or adding richness and shine to any color. That being said, if you already have very dark hair, you’re definitely limited in what you can do with demipermanent color. In this case, blondes do have more fun.
The low-level developer gives the demipermanent color some staying power (semipermanent dye is typically applied without any developer), but a demipermanent dye job is only expected to last for about 24 washes before that color coating fades away.
Benefits of using demipermanent hair color
Colorist Rex Jimieson says demipermanent color has five major benefits over permanent color: “It has no ammonia, it’s easier to grow out, it adds more shine, it’s easier to change later, and it processes quicker.”