Hey so to help you out I have actually kind of dealt with this problem before, and overcame it so to help out a fellow scripter i thought I would jump on this.
One thing that helped me when I made something similar to this which was a curved RPM line for my drag car was that I had a imagelabel behind it servering as the blank area that is just the unfilled shape.
After that I put a frame inside of the imagelabel and turned clipdescendents on for that frame, after this simply add another imagelabel and make that the red one. Let me know if that helps you out if not I can offer up an alternative way of doing this.