I have a 72 year old relative with almost only colored hair on his head. His hair has gone from mostly gray to mostly colored after I introduced him to specific supplements.
It seems it's a mix of the amino acid acetyl-carnitine and the vitamin nicotinic acid which is restoring his hair color.
Both of these supplements remove old mitochondria and increase the production of new ones. They also are anti-inflammatory.
As an added bonus, he now also has more hair on his head than he had before, with more and more new hair growing each month.
At least 1g acetyl-carnitine and 1g nicotinic acid. Higher dosages seem to work better. And they need to be taken together to work.