Hair dye stuff is able to partially penetrate the hair shaft. For this reason, the colour will endure recurring washing, typically 4-5 shampoos. It comprises no or very low grades of developer,peroxide or ammonia and are therefore safer for damaged or fragile hairs. Although, it may still comprise the toxic hue aggregate P-Phenylenediamine or other such components.
The last hue of each strand of hair will count on its original hue and porosity, so there will be subtle variations in shadow across the entire head. This gives a more natural result.