This is not a bug in dxfWriter, it was the original coloring in the Blender file...
New version online with experimental coloring from the rendered image.
Download dxfWriter 0.0.4
Note, the results from this are highly dependent on the blend file. Best results are achieved when the edges are inside the texture of the objects.
If the edges are at the outline, the result is mostly not so good...
Also the result varies at different sizes, I suggest 1024x1024.
This was a quick hack/test, so please report your results, maybe with attached .blend files... :-)
ILD SOS
Blender