Can't set background color of UITableViewCell in IB

To change the background color of the cell, you have to do it in the willDisplayCell:forRowAtIndexPath: method. This is mentioned in the UITableViewCell documentation near the bottom of the Overview. So you need:

- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath
{
    cell.backgroundColor = [UIColor redColor];  
}

The contentView is just the recommended subview to put custom controls in so that the cell gets layed out properly when table editing is going on.