Pencil, or graphite, is a medium that I'm drawn to, often. I can lose myself in the textures, shadows, highlights and shapes that make up the finished piece. I can focus on the subject and the personality I'm trying to project. A lot of times, when working in color, I get wrapped up in color shades and hues and lose contact with the subject, itself. With graphite, It's easier to stay, "In character" with the subject and not get sidetracked with all the technicalities that go into an oil painting or a watercolor.
Sometimes a subject, just doesn't need color, to stand on its own.
Ansel Adams had no trouble working without color, did he?