Most are shampooed on. Modern applicators make it easy to apply the colour just where you want on the roots where the new hair growth is showing. Permanent dyes are harmful to the hair, as they penetrate the hair cortex layer with the aid of hydrogen peroxide, destroy the keratin structure and create porosity. These dyes gradually eat away and destroy the delicate structure of the hair. This is true of semi-permanent dyes also.
Herbs are comparatively safe and healthy, and have been used since long a colouring agents.
The best known of these is henna i.e. mehndi . It is not only applied by Indian women on their hands and feet, but also used for colouring hair. Being a natural colouring agent, its use is not restricted to the young but also to the old. Henna is completely organic and comes in a powder form. Henna can do more than just colour the hair.