Hi Eduardo Trápani,
The fill issue is one that must be handled. Unfortunately, I do not
think any of the standard font formats can handle 2 color fonts
properly. There is no straight forward and correct way to do this.
You may want to make a separate font specifically for the fill
components. Maybe Graphite or some other font engine could combine 2
fonts when visualizing the characters.
I know 2 color fonts are an issue for another script beside SignWriting
but I can't remember the name. Maybe something has been accomplished or