Here's a quick class demonstration in Photoshop while teaching at California State University Northridge last year. Let's start with the end result and go backwards.
Instead of toning the composition with grays I went directly to local color.
The first step was in finding the color I wanted and then blocking it in.
This is where it began. The drawing was done digitally.
In a short period of time you can give your sketch that special touch by using it to introduce color in a direct and simple manner. Something nice for your portfolio or as a blog entry.