Demi-permanent hair color can lighten hair slightly and lasts between 12 and 26 washes.  It's a good choice if you are coloring your hair for the first time, and you don't want to lighten it too much.  It is best to choose a shade of demi-permanent hair color that is close to the current shade of your hair. This type of hair color doesn't contain ammonia and has less peroxide than permanent hair color. It also doesn't leave a visible root line. It provides 50 percent coverage of gray hair.
Permanent hair color, on the other hand, can lighten your hair two levels.  Because it penetrates the hair cuticles, the color lasts until it grows out, and roots of your natural hair color are evident within four to six weeks. To cover hair that is more than 25 percent gray, permanent hair color is recommended. 
All of that being said, if you have never lightened your hair before, you might want to try the demi-permanent color first to see if it provides the desired result. If not, consider using permanent hair color.