Permanent hair dye is the only way to have even colors throughout the whole head of hair, and the only way to disguise white or gray hair strands. In fact, if you have dark hair like black or dark brown, it is also the only way to color it into a lighter color such as blond or yellow, and bring out colors such as bright red or blue.
However, permanent hair dye really is long lasting, and you may want to give serious thought to the color that you want. Although bright blue with green streaks may seem like a rave idea at the moment, but once the deed is done, you cannot undo it. The colors will not wash out, although it may fade over time. Trying to undo the permanent coloring by dyeing the hair all over again with a different permanent color will definitely cause serious damage to the hair, that may only be salvaged by cropping it short or going bald, and waiting for the new hair to grow back.
This is because the hair coloring is actually done in two parts, the hair is first bleached to lighten the original color, and then the new color is applied onto the lightened hair. Once the permanent hair dye process is done, the only way to get back your original color, is to bleach the hair again to strip it of the dye colors, and then apply another dye which is similar to the natural color, to color over it again. This is a very harsh process for your hair, and very damaging too, and may result in weakened hair strands, hair loss, brittle hair, split ends, and frizzy hair.
All permanent hair dye contains an alkaline agent called ammonia. During the permanent hair coloring process, the ammonia in the hair dye is mixed with the developer, also known as the oxidizing agent, which is usually hydrogen peroxide. The alkaline solution is able to penetrate deep through the hair fiber and strip it of its natural pigments right to the core of the hair strand. The molecules of the coloring then infuses the lightened hair strands with the new color.
Maintenance and touch-ups of the hair color has to be done every 4 - 6 weeks to cover the new hair regrowth. If you choose a permanent dye to color your hair, you should be prepared to pay monthly visits to your hair stylist. Permanent coloring is best done by professionals, else the results may be devastating.