A few days ago on September 6 I posted some black line sketches in a topic called Girl head doodles. The first sketch intrigued me so I thought I'd work it up a little more in Photoshop.
Here's the original drawing. I took the linework and duplicated it creating a new layer.
Next I dropped a simple shadow layer to help establish dimensionality and form.
Followed this up with some local color and more shadows to push the depth.
I colored her skin to soften the lighting effects.
Lastly I added some design around her eyes and nose for novelty.
For fun I took away the skin tone layer to see what she would look like without it.
A few minutes playing with color and tone enhances a line drawing and gives it more life. A benefit of this approach is you can stop at just about any place in the process and your art will still look good without overworking the subject. Or you can keep going and take it as far as you like.