How to fill with translucent color the space between two lines in Unity3d?

There are dozens of caveats to this, but the place to get started would probably be to build a Procedural Mesh of 2 triangles between the start and end points of each of those lines. Since both share a starting point, you could simplify it into a single triangle on that basis.

For efficiency, you'd just want to make sure you're using a List<> rather than an Array[] for vertices and triangles.