The main causes are external agents such as sun rays and pollution. Another cause might be a bad lightening process: if the hair pigments oxidation process is not completed, a reddish or yellowish color emerges. During a long sun exposure, blonde hair will have a golden yellow hue. UV rays can also turn colored hair into yellow, which usually have red pigments in it.
It is useful to contrast yellow tones which often appear on lightened hair, giving ash shades. It is used on grey or light blonde hair. It can also be used on red hair to remove copper or orange shades. It does not work on dark, black, brown hair. Both men and women can use it.