I've done as MarkMusicNYC stated and it has worked .... I use to use JFM and then noticed I can get 10 times as much for the same price if I got the one at the womens shop. I have very bark brown w/mix gray, and use the light to med brown leaving it only a few min. The results are act pretty good leaving just the right amount of gray mixed in with the browns. From what I was told, ones hair is actually different shades so the mixed color looks more natural.... I guess.
I'm a licensed barber that's been dyeing my beard for years now. It can actually look great. Here are the secrets to doing it right: