Reverse Color Key

I want to transparent everywhere except for one color (red) in a playing video. In the picture that I sent, The part I show is moving so I can't just crop it. How can I do this?

As far as I remember previous forum posts, this inverse color key operation never had a solution.

Keep in mind, everything except the red parts will vanish, including the white and black border of the circle and the black drawing within the red circle as well. This sounds not very usable.
In essence, you will probably want to create a dynamic mask with that function, where you want to replace the background with another source and the matching color with a third source. Not keep it red but replace that with even something different.