NYX Dark Brown is a medium-dark, milk chocolate matte brown. It has a slight warm yellow undertone, as opposed to a red undertone that you may find in some brow powders. This works great with my hair color and would work well for anyone with light to medium brown hair. For those with black hair, you may want a color that is slightly deeper.
I've been using it almost everyday along with my Make up Forever Brow Corrector. It is the perfect shade of ash brown. The consistency is a little powdery, and it is pigmented, but it's not too pigmented (where it makes your brows look painted on.) So when applied, it isn't harsh. It applies light and natural like a brow powder would.
Plus, it's inexpensive and will last you a long, long time. I would compare this to MAC Espresso, one of my favorites for filling my brows.
Here's a swatch on my hand..