What Makes Hair Dye Change the Color of Your Hair?

Hair Dye Basics

Changing your hair color can be a way of changing your appearance. Whether the dye is purchased at a drugstore or applied by a professional salon colorist (recommended), you can achieve various results. Hair dye either deposits over the color that is already there or removes the color that is naturally there to apply new, artificial color in its place. There are three types of hair color: semipermanent, demi-permanent and permanent.

Semi-Permanent or Temporary Color

Semipermanent or temporary coloring deposits direct color dyes into the first layer of the hair, called the cuticle. It covers the surface only, therefore changes the color without permanently altering the natural color underneath. Semipermanent color lasts approximately two to three weeks. For this application, no developer is needed.
A developer is made of hydrogen peroxide, a mixture of water and oxygen. When the oxygen is released into the hair, it breaks apart into small fragments, making the hair lighter or creating room for pigment. The oxygen develops the color and deposits artificial pigment in its place. A higher volume developer contains more oxygen and will develop the artificial color faster.

Demi-Permanent or Deposit-Only Color

Demi-permanent hair dye penetrates deeper into the cuticle, and will either deposit an oxidized dye or direct dye. An oxidized dye will not need a developer, as it already contains the oxygen that is needed to develop the color. A direct dye is a vegetable-type dye that contains color and may or may not need a developer. This type of hair coloring will last longer than semipermanent dye---typically four to six weeks. The results will be similar to semipermanent color.

Permanent Color

Permanent color can be both deposit only or lift, making the hair lighter. It is similar to demi-permanent, however a developer is required. The difference is that permanent color will go through the cuticle, deep into the cortex and can deposit, change or lighten the natural color that exists, leaving artificial color in its place. The cortex is where natural pigment lives. Unlike semipermanent or demi-permanent color, permanent color will not wash away because it is beneath the surface of the hair strand.
Permanent dye contains ammonia, which swells the hair, allowing the developer to enter the cortex and remove pigment. Hair color products with lower ammonia levels will cause less damage to the hair, creating a healthier, shinier look.

Gray Coverage

The best color to use on gray hair is a permanent dye. The result will last longer and the color will go deeper into the hair. If you prefer to simply blend the gray, a demi-permanent or semipermanent dye will work best.