I use Just For Men Beard and Mustache and it works fine, but there was a learning curve to get the results I wanted.
I first thought that medium brown was my color, but it was a bit too light and looked rather reddish, so I then tried dark brown which was a good match.
If left in for the full five minutes per the instructions, it goes pretty dark. Rinsing it out after three or four minutes gives a less drastic look.
Thoroughly applying it to the whole beard resulted in a mass of solid color, which looked unnatural on me. It’s possible to feather it in and leave some “salt and pepper” look, which I prefer. It takes a surprisingly small amount of product to do this.
It can also be applied to the temples for those want them to be less gray.