That Filter is really powerful in selecting a certain color. Unfortunately it removes everything but that color if you chose "Mask to alpha".
If you would be able to invert that mask, it would be a pretty perfect chroma key filter!

I know, there is already a chroma key effect, but that one always leaves a border... and you always need at least two effects to get a good result. Reducing that boder using that key spill... effect did never work for me properly. Sadly it is not really well documented...

you have to fill a FEATURE IDEA on phabricator... https://phabricator.kde.org/tag/kdenlive/ maybe in future this can be done.