Hair Dyes are chemicals that are used to change hair color.
Consumers considering changing their hair color have a choice of coloring agents to use. What distinguishes them is how long they last and how they color hair. There are at least four basic categories of products:
Temporary hair colors are applied in the form of rinses, gels, mousses and sprays. These products merely sit on the surface of the hair and are washed out with the next shampoo.
Semi-permanent dyes penetrate into the hair shaft and do not rinse off with water like temporary colorings. Semi-permanent dyes usually come in liquid, gel, or aerosol forms.
Permanent dyes require more work to apply, but the hair color lasts until the new hair - "roots" - grows in. Because permanent dyes contain hydrogen peroxide, they cover gray hair more effectively and can be used to lighten hair color, unlike other dyes.
To apply permanent dyes, the user mixes together a hydrogen peroxide liquid with another liquid, works the mixture into the hair, and after about half an hour rinses the dye out with water.
Permanent dyes not only penetrate deeply into the hair shaft, but get locked within it due to a series of chemical reactions that occur while the dye is applied. Consequently, permanent dyes generally cannot be washed out with shampoo, although repeated shampooing may reduce the color over time.
Gradual or progressive dyes are dyes in the form of a rinse that slightly darken hair by binding to compounds on the hair's surface. Gradual dyes are usually applied daily until a dark enough shade is achieved, after which it may be used less often to maintain the color. Unlike temporary dyes, gradual dyes do not wash off readily or run when the hair gets wet.