In 3D move I want to create a “flat” polygon plane shape and assign a color to it.
So just as an example I have built below shape with a Path and now want to fill the inner part of the polygon with a color:
What is the best way to achieve this? Needs to be done via script because I am loading the shape data from a CSV file.
I would try a Polygon2D. You can create one via script and set its polygon and color properties.
https://docs.godotengine.org/en/4.3/classes/class_polygon2d.html
In 3D, though, objects may not render properly if they don't have thickness. But I don't know how you 're using these objects.
DaveTheCoder I want to put the object on the plane surface to display a water body like a lake. But looks like I cannot easily add a Polygon2D to a 3D scene.
What about a MeshInstance3D? It wouldn't matter if it has thickness, if it's a body of water and only the top is visible.
DaveTheCoder ok but how do I build that polygon mesh?
Why don't you create the whole model with Blender and import it?
DaveTheCoder quoting myself here 
Needs to be done via script because I am loading the shape data from a CSV file
max_godot ok but how do I build that polygon mesh?
You can define a MeshInstance3D's mesh property via script. There's an example here:
https://docs.godotengine.org/en/4.3/classes/class_arraymesh.html#description
