Life used to be simple. You knew who your friends and enemies were, and there were three categories of hair dyes: Temporary, Semi-Permanent and Permanent.
* Temporary Hair Dyes -- are based on certified colorants and textile dyes that are too large to penetrate the hair and, hence, are removed by a single shampooing. Referred to as direct dyes, the colorants employed in these products possess an inherent color (i.e., do not require further chemical reaction to develop a color) that simply adds to that of the hair.
* Semi-Permanent Hair Dyes -- employ direct dyes based primarily on nitro-phenylenediamine that are small enough (less than 5Å1 ) to penetrate the hair shaft to some degree and, hence, remain on the hair through six to eight shampoos. As is the case with their Temporary counterparts, these systems simply add color to hair and thus cannot produce shades that are lighter than that originally present.
* Permanent or Oxidation Hair Dyes -- are two component systems with one component containing essentially colorless dye intermediates in a highly alkaline formulation and the other containing hydrogen peroxide.