The supplement that caused my hair to go from gray back to my natural “dirty blonde” is benfotiamine (150 mg per day). Benfotiamine is a relatively new oil-based form of vitamin B-1. It is not degraded by caffeine, alcohol or sugar, as is regular water-based B-1.
It was not fast-acting. It took about 6 months or so before color began to slowly return around the edges in back and on my side burns. I didn’t really pay much attention, but the color gradually (over a few years) increased in area to where it is now.
I take benfotiamine for it’s health benefits, the hair color is just a pleasant side effect.