If hair colour is a product of genetic inheritance, why does some people's hair naturally change colour over their lifetime (grey hair excluded)?

Hair and eye color are mostly determined by our genes. But it's not just by the genes we have, but also by whether those genes are turned on or turned off. And since genes can turn on and off throughout our lives, this means your hair color can change!

https://www.thetech.org/ask-a-geneticist/hair-color-can-change

It's not good form to offer a negative here but I called Dr Karl on a similar question why fair hair gets darker with age, which I also posted here at that time over 20 years ago. He had to admit he was stumped and I don't think anyone could answer here.

However the fact it tends to happen the most at puberty it turns out oestrogen and testosterone also increase melanin production in the hair, as well as its weight which increases.

https://www.glamourmagazine.co.uk/article/hormones-hair-colour