Fill Color and Opacity not affecting background image

So I’m trying to make a UI health bar(using a progress bar). That fades out whenever It reaches 100% aka full health. But when I try to change the Alpha in the Appearance Tab or if I try to create an animation for it, it only changes the opacity/alpha of the fill image, not the background image.

Example:

Imgur - http://imgur.com/xTE8j0s

Youtube - - YouTube