I've been making a few adjustments to one of my preference sets, and I'm just curious what "Color 0" means in the hard-coded layers in the General preferences.
Roband,
Colour 0 is a placeholder for layers where the assigned index colour doesn't matter. Layers assigned to index colour 0 usually are things like block layers, where the actual display colour of the linework inside that block will be determined by the layer that each line is assigned to.