The bars above the timeline are playback quality ... indicators. A yelllow line means Premiere thinks it might drop frames during playback, a red line means it expects that it will drop frames on playback.
I can think of one exception, Neil. The green bar. The green bar does indicate that the sequence is prepared for export via smart rendering. In that case, "green means go!" It is a nice guide to see that your sequence can use the render files for the export.