Let me show you a couple of small sketches I've enhanced with color fills.
The first step is to scan the image from your sketchbook OR photograph it (the poor man's scanner but it works!). All I did with this image is use the dropper tool to choose a color from the color wheel and then the bucket fill tool to click and fill areas with color. This works especially easily with black and white outline drawings. The color will stay inside the black lines and voila! There is an adjustment you can make to determine how sensitive the fill is--whether it fills all areas that are exactly the same color, or a range of similar shades. If the fill isn't what you envisioned, simply hit the back arrow to restore it to the last point.
Same thing here, but I also colored directly on the black lines of text as well.
In this final example I scanned in a painting and then began replacing the colors to create a visually arresting graphic. It's so easy to keep choosing new colors and filling areas to find the balance that seems just right. The fun with this image was how the color inside the collaged text ended up making the artwork. That was purely unintentional. I was just picking colors and overlaying them. Don't forget--you will still have your original image, so there is absolutely no limit to how many different artworks you can create from a single drawing! I like to print these final images out on fabric. What will you create with your enhanced sketches?