>>1692664Anything can be colored. Images that include shading give you a head start, and higher resolution cleanly inked images are easiest because you have more room to work.
Greyscale values from the linework image are simply converted into levels of transparency. Then you make layers behind the now transparent linework and use the lasso tool to make solid blocks of color for skin, eyes, mouths, hair, clothes, and so on.
Here's a decent overview of the process.
http://captainslug.com/actt/