The class you're using doesn't support it. Obviously the guy who made it (very generous of him, BTW. It's beautiful. I'm using it in my next app.) didn't need it in his app.
You have two options now:
Write code yourself/hope someone else here does that will do more custom drawing and allow you to set a separate border color. For someone with experience in the matter, it'd probably be a five minute job. I can say that I have never needed to use graphics contexts etc, so I can't help you there.
You could also email him and ask if he could extend the class. If you donated twenty bucks to him, he'd probably be more inclined to do it.
The source code with the original drawing code is here: https://github.com/PascalW/PDColoredProgressview/blob/master/PDColoredProgressView.m