Well, having developed a sensitivity to "phenyldiamine and other azo dyes" about 10 years ago, I was resigned to having grey hair. (I'm now 56).
However, I got fed up with it and decided to experiment.
10 teaspoons of dried walnut leaves
4 teaspoons of dried cheap tea
4 teaspoons of dried rosemary
1 litre of water (2 pints)
Boil and simmer for 20 minutes. Leave overnight.
Strain and add around 1 tablespoon of Irish Moss powder to liquid; bring to boil and leave to cool.
You should now have a very satisfying brown "gunge". If your hair is short, this will make 2 applications. It'll keep for up to a week.
Apply to clean, dry hair; cover with cling film and leave for about 4 hours. Rinse in lukewarm water until your hair "squeaks".
Yes, it's not dramatic but after 2 applications my hair was back to the darker grey of 10 years ago and even the man in the corner shop noticed!
By the way, the Irish Moss is a good conditioner and volumizer; if you have any Panthenol around the place add about 1% at the same time as you add the Irish Moss; this enhances the conditioning effect.
If anyone wants to "have a go" with henna, this is a good pre-darkener.
I'm going to invest in a grinder so that I can use the powdered leaves rather than a decoction; hopefully this will have a stronger effect.
By the way, dried chestnut leaves give a reddish colour.