The first two icons open a right-sidebar pane, but only one can be selected at a time (like tabs). Both can be deselected, however (unlike tabs). If either version of the pane is open, its respective icon is red, the active accent color.
The third icon opens a popover, and its icon is red not when the popover is open, but when an AirPlay speaker is selected.
The fourth icon reveals a horizontal volume control which covers the first three icons. The icon is never red.