As far as hair graying is concerned, I don't think that B12 does much unless you're seriously deficient in it.

There's some research in B-vitamins and PABA for hair repigmentation, reviewed (among other things) here: https://pmc.ncbi.nlm.nih.gov/articles/PMC6995950/

But the B-vitamin in question is calcium pantothenate (B5), and that B5/PABA research is very old -- from the 1940s and 1950s. The results were also hardly anything to write home about; a very mild effect at best.

I don't think that there is a reliable chemical agent to reverse hair graying at the moment, but it seems as though it's become possible in theory to target those mechanisms which appear to be responsible...