Permanent: Permanent hair color permanently changes the natural pigment of your hair. It contains ammonia, or similar ingredients, to 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: 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. It is also unable to make a permanent color change and the color usually fades after 28 shampoos. In addition, it won't be able to completely cover gray hair, although it will blend grays with your natural color.