A demi-permanet color lasts for about 28 shampoos, but, the color does not all wash out, it stains the hair and the idea is to lighten up the color just a little each time you shampoo, the only way to really find out is to have a conciltation on with a professional, we often book clients in my salon for just a consultation. As for putting a dark color in your hair and wishing to go lighter, you must use a color remover before you try to achieve a lighter color, then you most often need to apply a color to get at least something close to the color you would like. Just a suggestion here, but before I let my hair drow out to gtey, I would go to a wig shop and try on grey shades just to see what I thought of it.