It looks like the piece of code you provided is working as expected, Maybe the part that I don't see actually working is "hovering the filled portion", because right now the color changes on hover over any part of the slider.

In the case you want to do exactly the "hovering over the filled portion" you would need to build your own slider and make the filled and non-filled portions different elements.

If you can clarify maybe, I could help you more with this.