Change color of the progress bar at runtime in a transition

[Lehren]

I would like to change the color of the progress bar at runtime with a transition. For example changing the color from 'progress-bar-success' to 'progress-bar-warning' or even a custom defined color. Currently it's only possible to abruptly change the color by changing the background or setting the class.

A suggestion on a smooth transition on a color change would be welcome.