How do I use a PNG image as a transparency mask?

The screen I will be projecting on is in the shape of a rainbow. The screen is inset within the rainbow. The people supplying the projection gear have given me a PNG as well as a JPG in the shape of the screen to mask out all video that would show outside of the screen so there is no unintended bleed.

I have created a layer for this and moved it to the top of the stack. I have a Media Bin assigned to that layer with the image files within that Bin. The JPG gives me transparency, but the PNG does not even show up. The JPG’s black areas are transparent, but the white is opaque which is the opposite of what I need. Additionally, I am puzzled as to why the PNG is not working at all, but the JPG is and giving me transparency which to my knowledge it shouldn’t.

Can anyone help me get this setup correctly so everything outside of the viewing area shows only as black?

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sorted, it appears I was trying to combine two different methods. Once I stepped back and kept things simple, I got it figured out.