My buddy tried using Grecian Formula, and said apart from doing a nice job of regulating his natural color, it was the best hair dressing product he had ever used, making his hair very smooth and controllable.
Might be a good first thing to try, if you haven't tried or currently use, any rinses, or other coloring products. It only works on natural gray hair, not color-rinsed, or dyed hair that is starting to gray again by fading
Otherwise, you'll have to wait till your hair goes totally gray again, or grows out.
I use Clairol Men's Choice, every five or six weeks. (A rinse) Very natural looking results.
And I split each of the two bottles into two equal portions, which save dough. (I have short hair, so the bottles can go twice as far.) Price per box at Wally - about eight bucks.