Source and target colors of arcs with fields

**Is your feature request related to a problem? Please describe.**
Arcs can have a source and target color, but they are hand picked, or they can have a color for the whole arc by the values of a field

**Describe the solution you'd like**
That both the source and target color can be inferred from two fields of the layer

**Describe alternatives you've considered**
None

**Additional context**
None