There’s so many different ways to go about this, and this may not be the most efficient, but since your outline only needs to be black, you can just multiply all colors by the desired fill color and the black ones will stay black, and the rest will modulate with their own colors, so anything white will become 100% the active color.
In the case of your svg, after you load it into a TriMesh but before you create the batch, you’ll need to set all the vertices that need to change color to white (or if editing the source SVG is an option, just make them white in the original vector art)