There are three sets of lines - each in a different color. What do they represent?

When an electrode becomes loose or experiences other problems, those leads that are dependent on that electrode will manifest artifact. The colored lines indicate those leads that share a common electrode.

If the right arm lead is affected, leads I, II and aVR will manifest artifact.

If the left foot electrode is affected, leads II, III and aVF will manifest artifact.

If the left arm electrode is affected, leads I, aVL and III will manifest artifact.

If you are familiar with these three patterns, it will be immediately obvious to you that there is a limb electrode problem as well as the specific electrode causing the problem.