So in QCAD a black line gets rendered white, if the background of the drawing surface is pure black (zero, not one, two...255). I prefer drawing on a silver board (192). Background colour is an "application" preference, not a "drawing" or document preference and is not saved in the DXF file.
One could say, the DXFormat is underspecified for omitting such a preference. Or is it just QCAD? I rather draw draw black lines than those candy coloured lines I see so often. Still, other people have to look at my drawings with their software, and autocad is wide spread. Is there another way to help them, than writing with a white pencil a hint into the area, like:
"Please use a light background to view this drawing correctly."