This is a tutorial in which I explain how to paint your drawings in gimp. It is based on the subtractive fusion of colors available in Gimp and almost any other serious editing software.

Where to start?

Well, the first step is to have a defined sketch of what do you want to paint. I'm talking about real drawings, i. e. physically draw drawings. As we are gonna scan the paper we can sketch with a hard pencil or mechanical pencil (2H or H should suffice, it's a matter of taste) and then finish off with some soft pencil like B, 2B, 3B or even a black felt-tip pen (again, is a matter of taste).

Scanning

In the scan process, or later in gimp, we can edit the threshold where the sketched lines (which are lighter, i. e. "more white") disappear while the polished lines (which are darker, i. e. "more black") stay. If you plan to edit this in Gimp keep in mind that you should scan at gray scale (8 bit), do not use lineart (1 bit) as obviously you won't be able to adjust at all the threshold. We are aiming at a span of colors (gray shades, actually) which discriminate lighter grays while keeping the darker grays, an scanning at 1 bit per pixel depth produces images in black and white, no shades of gray rendering this step useless.

There are rules about how the resolution of the scan should be adjusted according to the final medium. As we are going to edit it extensively I recommend to adjust it to a medium/high resolution like 300dpi.

Processing the curves of the image

In Gimp, Colors->Levels... comes handy. (elaborate more: how is it done?, shots, et. al.)

Setting the layers and color blending modes

We are working with a grayscale image. This means that any color on the image has to be either white, black, or a shade of gray. So firstly we have to change the color depth to RGB in Image->Mode->RGB. This will transform the 8 bit image into a 24 bit one.

Now create a transparent layer on top of the image. Change the blending mode from "Normal" to "Multiply". Now, start painting on this new layer and you will see how the paint doesn't cover the background but instead it blends with it like we were inking our drawing.

In this mode, what we paint on the top layer is "added" to the bottom layer. Actually, we are not adding but rather subtracting. Subtractive colors work by subtracting colors (different wavelengths) from a white source of light. Think of the canvas as a white light bulb and of top of it we add different layers. If one layer is black, no light passes. If the layer has no paint (or, in Gimp, you paint with white), all light passes. If the layer has some ink, only the parts of the source light (colors, wavelengths) in accordance with the color of the ink passes. That's why it's sbtractive because every layer acts as a filter which takes "a part" of light from the whole white light. Black takes all, white takes none, and a color takes some part from the whole visible spectrum (the white light).