Sprite Filling not Working as Desired

So I'm trying to make a Health Bar by overlaying a filled sprite image on top of a background image.

The issue I am having is that the extremities (the first 20% and final 20%) of the filled image are a different color than the middle of the image, and also taper into points.

How can I get a filled sprite to have uniform size and color as it scales from 0 to 100%?