If I understand you correctly, you want to have a sold colour on the screen with a shape moving around, and as the shape moves it reveals the clip under it. Kind of like a shinning a torch* around in a darkened room. Right?
- Create an image of the sold colour and you cut out the shape that will be revealing stuff. The cut out should be transparent. Save it as a PSD.
- Then you import the PSD and place it on the timeline on the topmost video layer. Premiere should have auto-set the transparency settings for you, but if you want to check the key type should be set to Alpha-Channel.
- Now use the motion controls to move the image around the screen.
- Render and enjoy.
NOTE:
The image you create will need to be much larger than the video resolution; otherwise as soon as the edge of the image enters the video area the clip underneath will start to show. To work correctly your image will need a resolution that is three times (3*x by 3*y) your video resolution. This will also mean that you should tell Premiere to use the images aspect ratio, not the videos.
Think of it like a big piece of card with a hole cut in it, and you are moving the card around.
* A torch is a flashlight.