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
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
For example I am reducing the thickness of the line by painting with a brush.
Step 3: Add Alpha Channel & Threshold
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
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
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
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
If you like you can download the complete file with the layers so you can have an idea about how it is formed.