Semi-permanent hair dyes generally mean that they can withstand 6-12 shampoos before they fade. Semi-permanent hair dyes are applied to the hair. After 20-30 minutes, rinse with water to color the hair. The principle of action is that the relatively low molecular weight dye molecules penetrate into the hair epidermis and partially enter the cortex, making it more resistant to shampoo cleaning than temporary hair dyes. Since it is not necessary to use hydrogen peroxide to damage the hair, it has become more popular in recent years.