Change material inside instanced prefab, gameobject.renderer is NULL

I use an xml file to keep track of which prefabs I want to assign to which gabeobjects, which I instantiate at runtime, so the level gets generated dynamically.

So I load the prefabs with this line of code into my asset libary:

 GameObject roomCornerTile = Resources.LoadAssetAtPath<GameObject>(roomCornerTilesPaths[i]);

I set the GameObject var of one of my objects with this line of code later on:

 return GameObject.Instantiate(roomCornerTile) as GameObject;

After that, I perform transformations on it to place it in the actual world. Note, that the prefab and materials are loaded like it should and everything works as intended.

Now I want to replace the Material channel 0 which contains a transparent material by prefab definition so I can set it to a colored material according to my needs. This happens here:

 blueprintValidMaterial = Resources.Load ("Materials/TestMaterials/BlueprintValidMaterial") as Material;
 tile.mesh.renderer.materials[0] = blueprintValidMaterial;

Problem is, that exactly at this point, the renderer is NULL and I can't modify any materials or assign new ones.

Question:

How do I have to instantiate a prefab and modify it so I can change materials from an instanced prefab gameobject?

Here you can see the inspector of the prefab and the instanced gameobject. Note that the instanced object has a renderer in the inspector, but is missing the 2nd material channel.

I could not come up with a solution, please help!