iOS 8, 9, 10, 11…

The only way to set any color (with any alpha) is to use backgroundView:

Swift

self.backgroundView = UIView(frame: self.bounds)
self.backgroundView.backgroundColor = UIColor(white: 0.5, alpha: 0.5)

Obj-C

self.backgroundView = ({
    UIView * view = [[UIView alloc] initWithFrame:self.bounds];
    view.backgroundColor = [UIColor colorWithWhite: 0.5 alpha:0.5];
    view;
    });

Responses to Comments

• None of these other options reliably work (despite the comments below)

  // self.contentView.backgroundColor = [UIColor clearColor];
  // self.backgroundColor = [UIColor clearColor];
  // self.tintColor = [UIColor clearColor];
  

• the backgroundView is resized automatically. (No need to add constraints)

• Control alpha with UIColor(white: 0.5, alpha: 0.5) or backgroundView.alpha = 0.5.
  (of course, any color will do)

• When using XIB, make root view a UITableViewHeaderFooterView and associate the backgroundView programmatically:

  Register with:

  tableView.register(UINib(nibName: "View", bundle: nil),
                     forHeaderFooterViewReuseIdentifier: "header")
  

  Load with:

  override func tableView(_ tableView: UITableView,
                          viewForHeaderInSection section: Int) -> UIView? {
      if let header =
          tableView.dequeueReusableHeaderFooterView(withIdentifier: "header") {
          let backgroundView = UIView(frame: header.bounds)
          backgroundView.backgroundColor = UIColor(white: 0.5, alpha: 0.5)
          header.backgroundView = backgroundView
          return header
      }
      return nil
  }
  

↻ replay animation

► Find this solution on GitHub and additional details on Swift Recipes.