Each color is for a different muscle group: both yellow go on the same muscle group and both blue go on a different muscle group. Active or inactive electrode depends on its position on the muscle, but it doesn’t matter whether the electrode is marked + or -. This is explained in the section.
As a consequence, if your electrostimulator has a symmetrical waveform, you can swap the red lead with the black lead and nothing would change.
I hope this responds to your question.
Giovanni Ciriani