The problem with dark brown hair is that once you start coloring it, even a darker color, it will get lighter as the color fades. I know this from harsh personal experience
My hair has a lot of red in it and gets really brassy if I do anything at all to it.
The best thing is to find color that doesn't strip the natural color out of your hair and I have not found anything in the drugstore that does this. The "semi permanent" ones like Natural Instincts are just as bad as "permanent" color for me and they all fry my hair at least somewhat. People love Robert Craig color (available online) which is gentler, but that didn't color my gray well.
Henna (the 100% pure kind) is good for coloring gray, but it is red, so you would have red highlights. It is also time consuming and kind of messy. On the plus side it doesn't strip color from your hair so if your hair is very dark and only has a little gray it works nicely. You can also use indigo+henna to get brown instead of red, though the indigo will fade over time and the red won't--henna is very permanent. Recently I've experimented with Surya "hennas", which come in a variety of colours and have mostly natural ingredients. I was hoping to do brown or blonde highlights, but I've discovered that only the redder colors cover gray for more than a couple of washes.
Right now I am trying a dark ash brown I ordered from eSalon and it covered the red and the gray really nicely. Now to see how long before it fades and gets brassy.....
It's hard to find something strong enough to color gray, that won't lighten your natural dark color. I hope you get some good answers as I could use the info, too!