What is the difference between permanent and semipermanent haircolor ?

Semi permanent hair colors do not damage the hair and last from 4-6 weeks. They gradually fade out until the hair is the same shade it started from. These hair colors are deposit only, that is, they may temporarily darken but wont lighten the color of your hair. Since they do not damage the hair, you may reapply them as often as needed. The only downside is their limited ability to cover gray hair. 

Permanent hair color permanently changes the hair color. Your own hair color must grow out to fully remove this process. It can be reapplied as substantial outgrowth of original color becomes noticeable. However, each application damages the hair further, so it is preferable to apply the color only to the new hair growth. Permanent colors can either lighten or darken the original shade. They also cover gray hair extremely well. The downside is that you must grow the hair out and trim it off to remove it. It can sometimes be dyed back to match the original color although it is extremely difficult to match the shade exactly. This is when a professional colorist really earns their money!