It looks like they bright the line art forward with photoshop or what not. Then colored it digitally as well as the background was just a flat surface to see it better and get the right color for the skin.
yeah. it makes sense. when I started to draw that guy, it actually did look like a regular dude with horns. think of it as more of a caricature, even if you're going for a serious look. the eye brows, cheek bones are exaggerated and emphasized. the jaw sticks out further, and the smile is much larger and exaggerated. hope that helps a bit