These images are from a class demonstration while teaching at Cal State University Northridge in 2010. The lesson was in a visual development capacity and concerned value / tonal relationships and how they relate to color.
Started with a concept sketch drawn with a blue ink pen.
After scanning the sketch I began by laying out tone in a series of transparent layers in Photoshop.
I flattened the tonal layers and continued with new transparent layers working toward the darks.
Spent the last few passes of tone adding more contrast between lights and darks.
Finally I added color tone with transparent layers while blackening the background and overlaying the original line work after converting it from blue to black.
Final result is a composition that greatly embellishes what was originally a simple line sketch.
Even an obscure doodling can become compelling and dynamic when tone and color are utilized to bring forth the image in its dimensional aspects. This is a relatively easy approach to coloring in Photoshop that only requires some of the software's most basic tools. An artist with nominal familiarity of digital illustration software can make the most of what they know through this elementary technique when the focus is on artistic fundamentals. Emulate a traditional glazing approach and you'll get convincing results with your digital work.