title: Changing background color of not highlighted cell in UICollectionView
body: <p>I managed to set the cell background color when it is tapped on with this code:</p>
  
  <pre><code>- (void) collectionView:(UICollectionView *)collectionView didHighlightItemAtIndexPath:   (NSIndexPath *)indexPath
  {
      NSLog(@"highlighted cell at index path %ld", (long)indexPath.row);
  
      UICollectionViewCell* cell = [collectionView cellForItemAtIndexPath:indexPath];
      cell.contentView.backgroundColor = [UIColor blueColor];
  
  }
  </code></pre>
  
  <p>but then I have 2 problems</p>
  
  <p>1: I show 6 cells in a view, but then I slide to show the other cells, some of the cells are also highlighted with the blue background</p>
  
  <p>2: After I select cell A and have the background color changed to blue, when I select B, the background color in A does not changed back to white, how could I change it back?</p>
  
  <p>Thanks!</p>