Since the last time I used Faber-Castel "Pitt" brush pens, they've increased their colour range markedly, including a range of greys. I bought a Warm Grey III to try it out, along with a black and a sepia pen. I quite like them, though they don't have the response or size-range of proper sable brushes.
|Black and Warm Grey III|
|Dark Sepia and Warm Grey III|