The progress of the ProgressBar is actually a Rectangle that changes its size dynamically. You can customize the Fill of this Rectangle, or the Background of the RadProgressBar itself. Could this fit in your scenario? I've edited the Fill of the Progress Rectangle in the attached project.
Or maybe you need different Fill depending on the Value of the ProgressBar. If so, you can bind the Fill of the mentioned rectangle to the Value of its Templated Parent (the RadProgressBar) and use Converter to convert the Value to a different Brushes. Please let us know if you need further assistance on this.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items