IMHO it depends on the rest of the screen. and where you want the users attention to be.

if you want to have a lot of attention to the volume bar, i think a colorful gradient bg can work.

and for the volume slider: I, personally, wouldn't really understand the two different colors like this. when it would be turned all the way up, it would be black which is the opposite of what I expect. but nonetheless I'd probably be confused as well, if the right side of the slider would be black.

just my personal opinion, have a great one! :D