Hair Coloring 142 In recent years, the variety of available product forms has increased fairly significantly and now includes hair sprays, mousses, lotions, and gels. The products are sold either as low- level gray coverage with no hair damage, or for a temporary color change, e.g., to teenagers looking for a short-term vibrant color. The time required for color uptake is less than for the semi- permanent products, especially with the basic dyes, so the dyes can also be delivered from rinse-off products such as shampoos and conditioners. These “color booster” shampoos or conditioners may be included in retail coloring kits with an oxidative coloring product to be used between colorings. As the color fades, the shampoo delivers a boost of the tone to compensate. Natural Dyes Products containing natural dyes are less prominent in the current market due to their limited shade range and often long and messy dyeing process. In addition, they are often not compatible with the bleaching process, so the color change is limited. They have been all but replaced by semi-permanent dyes. The natural dye Henna has a long history of being used to color hair, and records exist of its use in the Bronze Age and during the run of the Roman Empire. It gives a bright orange/red color on hair. The active dye is lawsone (2-hydroxy-1,4-naphthoquinone) which is extracted during dyeing from the crushed leaves of the Henna plant (Lawsonia inermis). Indigo is a blue pigment that has seen use since the nineteenth century to color hair black. It is not very soluble in water, so it deposits on the surface of, rather than inside, the hair, and it is thus not very wash resistant. A series of example natural dyes, shown here and including the discussed Henna and indigo, mostly produce weak colors and are unstable if isolated prior to use. Melanin precursors have also been considered for hair coloring but the products have not been very successful in the marketplace. 5,6-Dihydroxyindole26 and 5,6-dihydroxyindoline27 and derivatives have been utilized in a limited range of products where air oxidation