For locks that are about 20 to 25% gray, use a demi- or semi-permanent hair color that’s closest to your natural tone in order to blend away the grays with minimal damage to the hair. Both types deposit color without ammonia, but the demi-permanent color "uses a stronger concentration of hydrogen peroxide to open up the hair cuticle and inject more color," Perry Romanowski, a cosmetic chemist in Chicago, told us. As a result, demi-permanent color lasts almost twice as long as semi-permanent. Semi-permanent color delivers a rich, shiny color but fades out in six to 12 shampoos.
If more than half of your hair is gray, go the permanent color route. Permanent hair color contains ammonia which opens the hair cuticle to allow the color to penetrate.
Many colorists advise not taking on the challenge of coloring gray hair yourself -- at least not the first time. Keep in mind that if things go haywire as you DIY, the color correction can cost more than a regular salon visit for coloring.