<P> Demi - permanents are much more effective at covering gray hair than semi-permanents, but less so than permanents . </P> <P> Demi - permanents have several advantages as compared with permanent color . Because there is essentially no lifting (i.e., removal) of natural hair color, the final color is less uniform / homogeneous than a permanent and therefore more natural looking; they are gentler on hair and therefore safer, especially for damaged hair; and they wash out over time (typically 20 to 28 shampoos), so root regrowth is less noticeable and if a change of color is desired, it is easier to achieve . Demi - permanent hair colors are not permanent but the darker shades in particular may persist longer than indicated on the packet . </P> <P> Semi-permanent hair coloring involves little or no developer, hydrogen peroxide or ammonia, and is thus less damaging to hair strands . The reduced amount of developer, whether peroxide or ammonia, means that hair previously damaged by applying permanent color or permanent reshaping is less likely to be damaged during the color application process . </P> <P> Semi-permanent hair color uses compounds of low molecular weight than are found in temporary hair color dyes . These dyes penetrate the hair shaft only partially, because of the reduced amount of developer used . For this reason, the color will survive repeated washing, typically 4--5 shampoos or a few weeks, before undergoing significant fading or washing out entirely . </P>

What is the purpose of hydrogen peroxide in hair color