Graying can occur for various reasons. Lack of copper or the amino acid phenylalanine are the most common reasons.
Rosemary, fo ti (he shou wu), and sage leaf are some of the most effective herbs for restoring hair color naturally. All are taken internally in capsules or as a tea. It will take a long time to see real effects as the hair already grown out is dead. So it is only the newer hairs that will eventually get color back, and this has to grow out.