[Cin] Color erase like GIMP

[Andrea paz]

In order to have a half transparency you could try to use Blue Banana
(see https://cinelerra-gg.org/download/CinelerraGG_Manual/Blue_Banana.html).
Then use Reroute plugin . I've often tried to create a track with only
the alpha channel that I could manipulate independently and then merge
via overlays, also using the F_separatefields plugin; but I've never
succeeded.
Last tip: when dealing with the alpha channel it can be convenient to
render a sequence of png (or OpenEXR) instead of just video formats.