Derek;

Color coding of insulator denotes wire gauge range of crimping ferrule (so indirectly correlates to current range), but this applies more for loose terminals used onezie, twozie, as a visual aid to the occasional installer. In a harness production environment, where crimps are fed from a roll into a pneumatic crimping tool, operators are less concerned about this...they make all crimps of a certain gauge , then move on to the next gauge. I think the black sleeving which is used on Volvo harnesses (which doesn't occur everywhere, was installed for mechanical strain relief in places where extra mechanical support was deemed necessary.

Cheers