Perhaps both conductors on one cable are earthed whilst the other cable has both connected to many volts - the shortest path for each to earth is the opposite wire?
As for blue / purple - maybe your monitor calibration is out or it just seems a purple colour in contrast to the blue background?
Why someone would wire both conductors in a cable to earth or high voltage is another question entirely!
<bloody pedants!>
Biting the hand that feeds IT
