The "Back Color" of the Alarm Banner object is what is displayed when no Alarms are active.
By default, both the Back Color and Border Color are red.
You can change the Border color to anything else, and then un-check the "Border uses Background Color" box to allow the Border Color to be used.
Why there's a manual setting as well as an override, I don't know.
Every Alarm message also has a Background Color and a Foreground Color that are specific to the Alarm Message. By default the background is Red and the foreground is White, so everything on that Alarm Banner is consistent.
I think what you want is to just set the Background Color for each Alarm Message: because an alarm fills the whole Alarm Banner, it will replace the default "Alarm Banner Back Color" background.
Clear as mud ?