Hair Coloring 138 Direct Dyes Direct dyes are intrinsically colored, meaning they do not need to react in order to form color, and tend to be more intense and vivid than oxidative dyes, with very bright colors and high molar extinction coefficients. They are utilized in semi-permanent and temporary products where no lightening is required. In recent years, direct dyes have also been added to Level 3 permanent products to add vibrancy to the shades. For example, Basic Red 51 and Basic Yellow 87 can be added to a red Level 3 shade to increase the vibrancy of the red tone. The direct dyes are often not stable for long- term storage in an oxidative tint formulation, and so are separately packaged and then added subsequently to the tint upon mixing with the developer just prior to use. The azo dyes in particular will be reductively destroyed over time in a tint formula that contains reducing agents (sodium sulfite and ascorbic acid) necessary to prevent the oxidative dyes from forming color on storage. NHCH 2 CH 2 OH NO 2 O O OH NH 2 N N N(CH2CH2OH)2 H2N HC Yellow 2 Disperse Red 15 Disperse Black 9 Nitro Dye Anthraquinone Dye Azo Dye The three main chemical classes of dyes utilized in these products are nitro dyes, anthraquinones and azo-dyes, examples of which are shown. Substitution on the structures can change the color delivered and reduce the longevity of the dyes’ ability to remain on the hair to just one wash for temporary dyes and 5–10 washes for semi- permanent dyes. Recent innovations in the direct dye area have focused on ways to increase the wash resistance of the dyes and make them stable to oxidation, either with hydrogen peroxide at pH 10 or with a persulfate bleach product. The dyes shown below are a selection