How to create a mask for color correction?

How can I add a mask in Motion for a color correction effect which applies the effect only inside or outside the mask and leaves the rest of the picture untouched? Whenever I add a mask everything else outside (or inside) becomes black (alpha channel). My dirty solution is to duplicate the video without the mask and put it underneath my masked picture.

Is this the way I have to do it in Motion? In DaVinci Resolve Fusion I just add a mask to an effect and the effect is limited to the mask, leaving the rest of the picture untouched. So, the effect is masked, not the video itself. I think, After Effects is similar. But in Motion I must handle two video layers to get the same result, isn't it?