title: about collection view, the corner cell doesn't keep the backgroundColor even though i change it
body: <p>I'm making a collection view that has 6 * 7 cells and even though I wrote code such that I could change each cells background colour after declaring the <code>UICollectionView</code>; the cell background colour stays white. Other cells are not returning back to white background colour. Can someone tell me the reason why only this cell doesn't keep it's background colour?</p>
  
  <pre><code>override func viewDidLoad() {
      super.viewDidLoad()
      let longPressRecognizer = UILongPressGestureRecognizer(target:self, action: #selector(ViewController.longPressAction(_:)))
      longPressRecognizer.allowableMovement = 5
      longPressRecognizer.minimumPressDuration = 0.5
      self.collectionView.addGestureRecognizer(longPressRecognizer)
  
  }
  
  func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -&gt; UICollectionViewCell {
  
      let col = indexPath.section
      let row = indexPath.row
  
      let cell = collectionView.dequeueReusableCellWithReuseIdentifier("CollectionViewCell", forIndexPath: indexPath) as! TimeTableCollectionViewCell
      cell.backgroundColor = UIColor.whiteColor()
  
      if !ViewController.colorDictionary.isEmpty {
          for (indexPath, color) in ViewController.colorDictionary {
              self.collectionView.cellForItemAtIndexPath(indexPath)?.backgroundColor = color
          }
      }
  </code></pre>