title: Can't change the color of scn file (materials / diffuse.contens) , using ARKit and SceneKit
body: <p>I just made the project: Xcode: File > New > Project > Augmented Reality App.</p>
  
  <p>I want to change the color of the ship.scn to yellow but nothing happened. How can I change the color of the ship.scn ??</p>
  
  <pre><code>override func viewDidLoad() {
      super.viewDidLoad()
      sceneView.delegate = self
      sceneView.showsStatistics = true
      let scene = SCNScene(named: "art.scnassets/ship.scn")!
  
      // here, trying to change the color of the ship.scn
      scene.rootNode.geometry?.materials.first?.diffuse.contents = UIColor.yellow
      sceneView.scene = scene
  }
  </code></pre>
  
  <p>I tried breakpoint and po command to check out what rootNode contains.</p>
  
  <pre><code>(lldb) po scene.rootNode.childNodes
  ▿ 1 element
    - 0 : &lt;SCNNode: 0x1c01f8d00 'ship' | 1 child&gt;
  
  (lldb) po scene.rootNode.childNodes.first?.name
  ▿ Optional&lt;String&gt;
    - some : "ship"
  
  (lldb) po scene.rootNode.childNodes.first?.geometry
  nil
  </code></pre>
  
  <p>Why geometry is nil?? I can see colored ship.</p>