red or green bar under M symbol in audio track?

Hi,
I didn't find it in the manual nor in www: some rare times I get a red or green bar under the M in the header of the audio tracks in the timeline. What does this mean?
Thanks!
Thomas
I didn't find it in the manual nor in www: some rare times I get a red or green bar under the M in the header of the audio tracks in the timeline. What does this mean?
Thanks!
Thomas