To get a colored surface you need a mesh. You can specify a point (or several for more complicated borders) inside your line and then specify the indices.
An example where something similar happens is in the collection
Collection of examples from discourse.threejs.org
2018 SwingingPolygon
Remove wireframe.
Note that instead of .addAttribute now .setAttribute is to be used.
Instead of .setDynamic( true ) now the usage is to be indicated.