Okay, first, we need to scan our image into the computer. How you scan the image is up to you. I usually scan in grayscale mode, but you can also do it in color or lineart/black and white. It's recommended that you scan your image at a resolution of at LEAST 200 dpi (that stands for "dots per inch"). Once you have the image, though, you'll want to convert it to RGB mode. Your program may even have the option of different bit depths. If so, choose 24 bit color.
This is the image I'm going to work with: (note that I am using GIMP on Linux for this tutorial :P )
It's basically a very rough pen and marker sketch of Zoey in her long hair phase. Kinda innocent and inquisitive.
Now that we have our image and converted it to RGB mode, open the layer window. You should only have one layer at this point. Most programs give the default name of "background" to any new image that has just been opened or scanned. Change the layer name if you like (I changed mine to "sketch"). Then, duplicate the layer (you can do this in the menu by going to "layers" and selecting "duplicate layer"). The new layer should say something like "background copy" or, in my case, "sketch copy."
Then, BETWEEN the "sketch" and "sketch copy" layer, I make another TRANSPARENT layer called "color". So your layers window should look like this:
Okay. Next, go to the "sketch copy" layer and set the layer mode to "multiply". Doing so allows any color below it to show through white areas. I find this a lot easier than trying to separate line art.
Note that some people will tell you to just make a multiply layer on top of the sketch and color on the multiply layer. I used to do this. But there is a major problem with this: first off, it alters the appearance of the colors a little. Second, if you fill an area on a multiply layer with the color white, it won't show up at all. And in many cases, you may need it white to be showing.
Next, select the "color" layer. Whip out the selection tool of your choice, either the lasso tool (
) or the polygonal selection tool. The area you select should have small, moving dashes around it. These are called "marching ants". Note that if you want to add to the selection, pick the selection tool again and, while using it, hold down the "shift" key. If you want to subtract from it, hold down the "ctrl" key instead. This is pretty universal in most art programs. It should look something like this:
Next, grab the paint bucket (
). Make sure you have the color you want chosen. Then fill the area. Like so:
Continue selecting areas and filling them until you have finished. Please note, though, that it's usually a good idea to paint figures in the fore ground on a different layer than figures in the background or extreme foreground.
This is what the color layer looks like by itself when I finished.
This is the image with basic coloring completely finished:
Cool, huh? Now, go practice!