There are many ways to color an image.
This option has worked for me for inked images with good contrast and is the fastest way I have found so far to do this.
Gimp is a free program to manipulate images and drawings, similar to Photoshop. This program works by separating the image in layers. On one layer you put your black and white drawing. On a second layer you fill in the colors. On a third layer you draw the background, and so forth.
Note: You will find this tutorial in my blog www.fiestoforo.cl too, I have just shared it here.
Original and final version of the drawing.
Step 1: Open Your Black and White Image in Gimp
Before starting, you will need to download Gimp from http://www.gimp.org/ and scan your black and white drawing. I scanned this image in grayscale (resolution 300 dpi or higher).
Open your image in Gimp by dragging the file into Gimp gray canvas or by using the dialogue in the File bar as in any other program.
Step 2: Fix Any Error and Clean the Image
Prepare your image to be colored. Remove any sign of dust from your scanner. You can delete mistakes or add some parts in this stage.
For example I am reducing the thickness of the line by painting with a brush.
Step 3: Add Alpha Channel & Threshold
Check whether the image has an alpha channel, which allows transparency in the image. Go to the layer dialogue (must be on your right, under heading layers), and right-click the thumbnail of the image. If add Alpha Channel is available, select it.
When the image has an alpha channel, you can apply color function called Threshold, which converts the image into a black and white version, enhancing contrast.
Filling with color will be much easier if the image has a good contrast, with no shading areas.
Step 4: Select by Color and Remove White
Use select by color tool and click on an white area, then you have all white areas selected. When it is selected, you will see what is called "marching ants" (animation of a dotted line) around your selection. Click delete or cut (control + X).
You end up with an image which have the trace of your drawing and transparent areas. Transparent areas are represented by a checkered pattern.
Step 5: Duplicate Layer
Now we have been working on one layer, but to color we will need two layers.
The upper layer will have the black and white trace or drawing.
The second lower image will be colored.
Two duplicate a layer, just select it in the layer dialogue, and right click, duplicate or press the button that looks like two image frames.
Step 6: Paint With Bucket Fill or Pencil
Note: If you want to color this image you have to change its mode into RGB (a mode which supports color). Go to (Image/mode/RGB).
Coloring process is starting.
Paint by filling transparent spaces or islands.
(optional) Use a third layer for difficult parts.
When you want to fill several spaces or islands, you can use a third layer below color layer, and using pencil tool to color.
In the fourth image, i am filling the hand of the character which is surrounded by blue..
Step 7: Final
This is the final image, with background some clouds, and other details.
If you like you can download the complete file with the layers so you can have an idea about how it is formed.