Can't set the background color of paper-progress from javascript

[yogevyuval]

I have an element that contains a element. The is updated every second with ajax and get's new details. It get's a number between 0-100 that is binded to the paper-progress to show the current value. I wanted to have the progress bar to change color according to the progress value. I tried things like:

<paper-progress id="prog">....
this.$.prog.querySelector("::shadow #activeProgress").style.backgroundColor = "some color here..."

and

  this.$.prog.shadowRootquerySelector("#activeProgress").style.backgroundColor = "some color here..."

And nothing worked. It's like I didn't manage to really set the color or something was overriding my color choosing.

Is that some problem or did I do something wrong?