When your hair is dirty it is harder for the colorist to determine the exact color of your hair, whether it has oxidized, and if subtle changes might have occurred during the course of the touch-up. Clean hair enables your colorist to have a clearer picture of the proper adjustments that need to occur to keep your color perfect. Hair color also takes better on clean hair. If you do have a sensitive scalp and are worried about a reaction to the coloring process, gently wash your hair. Do not scrub your scalp.