Hi zoran1973.
Now listen, first I recommend this page. It explain in details the use of "Adaptive Local Thresholding" that you can use in ImageMagick "-lat".
The problem you are trying to solve is a classic "photocopy" like filter.
Try this topic: The recipe for "GIMP/Artistic Filters/Photocopy".
Finally, read these topics in IM Examples Usage it will be good for you:
Children's Color-In Outline Image
Divide (removing shading effects)