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It is quite fashionable to colour hair these days, not only because it helps to hide hair that is turning gray, but also to follow current trends. A trip through the local store aisles will uncover a wide variety of half colouring products designed to give results for varying lengths of time. Permanent hair colour products have high concentrations of peroxide and ammonia designed to break down the hair by opening or cracking its outer layer, to ensure that the dye penetrates to the centre of the thicker gray hairs for good. These products do not affect the colour of new hair growth, however, so there will soon be evidence of the gray against, close to the scalp. Semi-permanent hair colour products have lower concentrations of peroxide and do not contain ammonia. While they cause less damage, they do not completely hide natural colouring, generally providing about 80 percent coverage. The colour fades gradually with washing, and will usually be gone after about two months. Semi-permanent hair colour products are the safest and gentlest on the hair. Having not ammonia and very little, if any peroxide, these products will cover only about 10-50 per cent of the natural colouring and are often used for fun and entertainment. Their duration of coverage is about one to four weeks, depending on the frequency of washing. The colour can usually withstand about four to six shampooing sessions before it fades completely. Hair colouring of a less than permanent nature is generally used for entertainment because it can be shampooed out of the hair quite simply. Colour rinses fall in this category and are used to give some tone to the hair. These products normally have only one component, the temporary hair colouring itself.