.fbx Model grayed out can can't be edited

Question 1

When I save as .FBX and import into Unity 3D they become grey. What's the reason of this and how can i solve it?

I've gone through the same issue when importing the following 3D Data Visualization model

The model was created using Cinema4D and exported as .fbx.

When i imported into Unity using drag and drop, I got the following result

As we can see from looking at the Inspector (image above) the material is grey and can't be edited, the textures were not imported. The materials of the default assets cannot directly be edited but there's a way to go around it.

I've created a new material (right click in assets > Create > Material)

Gave it a new name (green) and changed Albedo to green.

And dragged the material to the cube

The end result now looks like expected

Also i have huge troubles with performance of model on mobile. What's the best approach to get app faster?

It can be tricky to develop mobile VR apps but the game window has an info panel that displays useful live information (Use Stats button in the Game window to enable it)

There we can read how many SetPass calls, Batches and polygons are being drawn. Aim for 50 SetPass calls, that's a good number accordingly to Unity experts.

About optimizing performance, some very good developers already explain in detail how to optimize your scenes. I suggest the following links

Question 2

1) all models gray- this means the texture was lost in the import. You can create materials and drag them onto the mesh. But Unity can import your max files directly, you dont have to export to .fbx. Maybe if you try that your textures and materials will be setup automatically. 2) performance on mobile- can be various causes. maybe too much detail in your models. Or more likely too many drawcalls, or maybe too heavy cpu upsage . hard to say. the unity docs have pointers on optimizing for mobile.