Dye hard with a valence

As we age, our locks grow grey and many people choose to colour their hair in an effort to hold onto the appearance of youth.

There is no natural way to recover lost hair colour, so many people with silver locks rely on artificial hair dyes. In Europe and the US, roughly three-quarters of women use a hair colourant – although this is not exclusively because of grey hair. In 2016, almost 1.43 billion units of such colour products were sold in the US alone. The global hair colour market was valued $18.6 billion (£14.2 billion) in 2016, and it is projected to grow at a compound annual rate of 8.6% until 2024.

But the science of hair colourants for grey hair isn’t as straightforward as it seems.