iOS UICollectionView show strange when cell has background colro

- (UICollectionViewCell *)collectionView:(UICollectionView *)cv cellForItemAtIndexPath:(NSIndexPath *)indexPath;
{
    static NSString *kCellID = @"Cell";
    WordCell *cell = [cv dequeueReusableCellWithReuseIdentifier:kCellID forIndexPath:indexPath];
    
    cell.label.text = [NSString stringWithFormat:@"%i%i", indexPath.section, indexPath.row];

   switch (indexPath.row) {
        case 0:
        case 1:
        case 4:
        case 5:
            cell.backgroundColor = [UIColor greenColor];
            break;
            
        case 2:
        case 3:
        case 6:
            cell.backgroundColor = [UIColor redColor];
            break;

        default:
            break;
    }
    
    return cell;
}