If you need to color code your wire to simplify your crossover construction, take your basic broad tip black Sharpie and put a stripe on the wire you will use for your negative run. A black stripe on the side of the wire from one end to the other has no negative effect on the sound. You may even be able to hear the effect of any dye at all in the insulation regardless of color when compared to insulation with no dye.
For some reason even teflon when dyed black is seriously degraded, with a grungy/dirty sound when compared to virgin teflon containing no dye at all.
Scotty