- Create a solid layer background color
- Create a solid layer foreground color (different colot)
- Create mask in FG layer
- Set background layer's track matte to be the foreground layer
- Create an adjustment layer above it all. Add "Set Matte" effect. Take Matte from layer "FG" Masks.
In output module, set color to Straight (Unmatted).
If you render the comp at the comp's native resolution, the render (h264 or ProRes etc.) will bypass the alpha and be "the full image".
If you enable the resize section, the render will necessarily be Straight (unmatted) (ie, black background instead of the desired comp).