Rotating mask to reveal circle

I need to have a circle of one filled color change to another filled color as if a clock hand was revealing the under color. It's purpose is to show progress. For example, the circle might start as filled green, and then as it progressing to 100% it's filled with red. The under color is revealed in the same movement and position as if a clock hand were revealing it as it moved past. I hope that makes sense.

I need a strategy for the masking or animation - not any actionscript to show progress. Thanks.