A new Volume Control was introduced with Windows Vista and continues on in Windows 7. In addition to the volume level slider that the user can drag up or down, there are 2 volume levels colored bright green and dull grey that animate in the background. What do these two levels represent?
The green and grey levels animate only when some audio is being output by any open application. The green level represents the current volume being output by any application as a fraction of the current volume level represented by the slider. The grey level on the other hand represents the current volume being output from any application as a fraction of 100% of the volume.
That is, if the user pulled the volume slider all the way to the top (100%), the green and grey levels would be equal. Since the green level is overlaid over the grey, only the green level would be visible in that case since it completely conceals the grey.
(Thanks to friend Poonna.)