title: Change color of SCNNode/Geometry/Material
body: <p>I am very new to SceneKit, and am starting a test application. I have a little cube that moves around. I would like to add axes (X, Y, Z). I have created them like so.</p>
  
  <pre><code>    SCNVector3 xPositions[] = {SCNVector3Make(-50, 0, 0), SCNVector3Make(50, 0, 0)};
  
      int indicies[] = {0, 1};
      NSData *indexData = [NSData dataWithBytes:indicies length:sizeof(indicies)];
  
  // X axis
      SCNGeometrySource *xSource = [SCNGeometrySource geometrySourceWithVertices:xPositions count:2];
      SCNGeometryElement *xElement = [SCNGeometryElement geometryElementWithData:indexData primitiveType:SCNGeometryPrimitiveTypeLine primitiveCount:2 bytesPerIndex:sizeof(int)];
      SCNGeometry *xAxis = [SCNGeometry geometryWithSources:@[xSource] elements:@[xElement]];
      SCNNode *xNode = [SCNNode nodeWithGeometry:xAxis];
      [self.gameView.scene.rootNode addChildNode:xNode];
  
      // Y axis
      SCNVector3 yPositions[] = {SCNVector3Make(0, -50, 0), SCNVector3Make(0, 50, 0)};
      SCNGeometrySource *ySource = [SCNGeometrySource geometrySourceWithVertices:yPositions count:2];
      SCNGeometryElement *yElement = [SCNGeometryElement geometryElementWithData:indexData primitiveType:SCNGeometryPrimitiveTypeLine primitiveCount:2 bytesPerIndex:sizeof(int)];
      SCNGeometry *yAxis = [SCNGeometry geometryWithSources:@[ySource] elements:@[yElement]];
      SCNNode *yNode = [SCNNode nodeWithGeometry:yAxis];
      [self.gameView.scene.rootNode addChildNode:yNode];
  
      // Z axis
      SCNVector3 zPositions[] = {SCNVector3Make(0, 0, -50), SCNVector3Make(0, 0, 50)};
      SCNGeometrySource *zSource = [SCNGeometrySource geometrySourceWithVertices:zPositions count:2];
      SCNGeometryElement *zElement = [SCNGeometryElement geometryElementWithData:indexData primitiveType:SCNGeometryPrimitiveTypeLine primitiveCount:2 bytesPerIndex:sizeof(int)];
      SCNGeometry *zAxis = [SCNGeometry geometryWithSources:@[zSource] elements:@[zElement]];
      SCNNode *zNode = [SCNNode nodeWithGeometry:zAxis];
      [self.gameView.scene.rootNode addChildNode:zNode];
  </code></pre>
  
  <p>My problem is that I would like to change the color on them so I can tell which is which. Is there a way I can do that, or do I have to create my lines another way? I'm making a mac app, not an iOS app, just an fyi. Also, I know about repeating code, I was just tired and i will fix that.</p>