For creating color v0 variable font I create variable font though fontra, then combine it with manually created CPAL and COLR table in ttx format using ttx tool and generate ttf files.
It would have been nice if fontra could read and write ttx format directly.
It would have nicer if it allowed direct editing of other tables of ttx like COLR just like it supports editing of features file.
Till date no font editor uses ttx as source format. But being XML it should be acceptable as a source format