I think there are lots of causes of gray hair - I mean that many different deficiences can cause gray hair.
But to answer your question - I attended a two week raw food retreat where we all drank wheat grass juice 1-2 times a day. We ate a lot of energy soup and salads. There was a man from Florida there who wasn't that old but had very short white hair. By the end of the retreat, his hair had many noticable black hairs.