Permanent hair color permanently changes the natural pigment of your hair. It contains ingredients that help the color molecules penetrate the hair cuticle and deposit permanent color. Because it is a stronger formula, permanent color provides the best gray coverage and also has the ability to lighten natural hair color.
Demi-Permanent hair color deposits color pigment onto the hair cuticle without penetrating it. This means that demi-permanent color can darken your hair color, but the absence of ammonia makes it unable to lighten the hair. Since it does not make a permanent color change, the color fades gradually, lasting up to 24 washes. In addition, it is unable to completely cover gray hair, although it will blend grays with your natural color.
You can easily apply a permanent formulation over demi-permanent color. You can also apply a demi-permanent over a permanent application. However, your hair should be less than 50% gray since demi-permanent does not cover grays as effectively as permanent formulations do.