Hi,
I want to know how it is possible with Unity to import a .fbx model, then pick a specific polygon of that model and then change its color/material?
How can I render the wireframe of the imported fbx model in game mode?
No, you can’t change color of specific polygon. Unless you do some scripting, removing that polygon from that submesh and create a new submesh with that polygon and desired shader/color.
If all of the tris are detached (no mesh smoothing), and you have a MeshCollider, and you have a shader that allows Vertex Colours, you can do it. However it’s probably going to be slow and awkward to accomplish. The idea being you raycast to the MeshCollider (that has the same mesh as the Renderer), you get the 3 indices that are referenced for that triangle using the raycastHit functionality, then you change the Vertex Colour for those indices. If there are smoothing groups those colours will bleed onto nearby triangles.
Unity - Scripting API: RaycastHit.triangleIndex has a sample of how to find the vertices of the triangle you hit.
After that you need to create a custom shader that allows for Vertex Colours.