One of the main reasons would be covering gray hair as well as white hair.
It's very popular to cover grey hair with hair colouring
I am thinking about coloring my hair. Why? Do you not like your hair colour? What colour are you thinking of changing it to? I like my hair , but friends are doing it so I want to fit in. Cool its very popular, in fact, 75% of the women in Copenhagen have used hair dye according to an University of Copenhagen study! 75%? That's crazy to think about! Are there other reasons than fashion for hair coloring?
The main reasons for this are cosmetic: to cover gray or white hair, to change to a color regarded as more fashionable or desirable, to restore the original hair color after it has been discolored by hairdressing processes or sun bleaching.