@SteveDee Yeah that's exactly what I did. Hint: cut the sheath longitudinally at first to guage the depth of the outer sheath. That way if you cut too deep the blade will probably end up between two conductors rather than cutting across them. I know there's no guarantee of wire colours in mass-produced USB cables but I've never seen one with non-standard colours myself (and I've seen a few!). In any case the 5v 0v wires are usually easy to spot as they are thicker, and it's easy to use a meter to check (lack of) continuity to ground with the cable plugged in and the power off.