Bastien asked to split this of bug 574312:

As discussed on IRC it might make sense to expose base_volume/VOLUME_NORM as colors on the slider.

Here are my suggestions:

A) if decibel volumes are supported both for sinks and for sources the volume
range should be extended to 150% or so. (We already have that for sources, but
I think this should be allowed for sinks, too. It is useful if you play some
movies on unamplified speakers and has been requested many times on the the PA
IRC channel, PA ML)

B) If decibel volumes are supported and base_volume is > MUTED and < NORM, then
the part of the slider between MUTED and base_volume should be colored green.
The part of the slider between base_volume and NORM should be colored yellow.
And the part > NORM should be colored red.

C) If decibel volumes are supported and base_volumes equals MUTED or NORM the
part between MUTED and NORM should be colored green, and everything > NORM
should be red. No part shall be yellow.

D) If no decibel volumes are supported, the range cannot be extended, and hence
is limited to MUTED..NORM. 

E) If no decibel volumes are supported and if base_volume is > MUTED and <
NORM, the part < base_volume shall be colored green, the part > base_volume
shall be colored yellow. No part shall be red.

F) If no decibel volumes are supported and if base_volume equals MUTED or NORM
the part entire slider should be green.

I'd assume that the coloring would be implemented as smooth gradient. If that's
the case then in cases E and F it might be an idea to start the gradient to red
at the end of the slider, but not actually become fully red, because the slider
ends at NORM.