Or, you could scan it into photoshop. Trace over your lines with the brush or pen tools, colour inside, dodge and burn or use transparent brushes for shading, and have the original sketch on a low transparency multiply or overlay layer. In fact, most paper sketches do get scanned and finished up in photoshop.
Hand-drawn art doesn't necessarily mean it's drawn on paper. I use a tablet, so everything of mine is still hand drawn.
Though if you want to try watercolour, go for it! His skin tone is a bit confusing, so good luck with that.