This is a class demonstration I did in 2007 when I was teaching at Cal State University in Northridge. Some of my students were having trouble getting started with a basic exercise in Photoshop for visual development geared towards enhancing one's production design skills. So I used this blue line sketch as an example of how to move forward in a simple yet effective way.
After scanning the above drawing I created a new layer and used it to color over the original image using flat shapes of tone to define the gray scale relationships of each area of the composition.
Next I created a transparent layer and used it as a means of tinting certain areas with color.
I took this a step further and added more color tints while softening highlights in her hair.
I could have gone further with rendering the composition but it worked out fine for the purpose of the class session. From what my students were doing afterwards I could see how this method gave them a footing on approaching color in a controllable and confident manner.