Comment 15 for bug 653446

The base volume is really not something that you can change. It's a product of what comes from the hardware. A lot of hardware does not provide this information and thus Base Volume == PA_VOLUME_NORM == 0dB == 100%

It is only really relevant for initial volume choices for new users and to display to the user that anything above the base volume can be subject to some degree of clipping/artefacts.

For reference, some documents about how to present volume sliders is present here:
http://pulseaudio.org/wiki/WritingVolumeControlUIs#Colouredvolumesliders

No-one has yet implemented such a coloured scheme (with the exception of VLC which uses a very similar mechanism, in it's native GUI, but with no direct relation to PA and base volumes. It also uses a completely different dB->percentage mapping to that used by PA)