IOS 8 Tab Bar Item Background Colour

In your tabBarController, you can set the default UITabBar tintColor, barTintColor, selectionIndicatorImage (cheating a bit here) and renderingMode of the images, see comments below:

    class MyTabBarController: UITabBarController, UINavigationControllerDelegate {
      ...
      override func viewDidLoad() {
        ...
        // Sets the default color of the icon of the selected UITabBarItem and Title
        UITabBar.appearance().tintColor = UIColor.redColor()

        // Sets the default color of the background of the UITabBar
        UITabBar.appearance().barTintColor = UIColor.blackColor()

        // Sets the background color of the selected UITabBarItem (using and plain colored UIImage with the width = 1/5 of the tabBar (if you have 5 items) and the height of the tabBar)
        UITabBar.appearance().selectionIndicatorImage = UIImage().makeImageWithColorAndSize(UIColor.blueColor(), size: CGSizeMake(tabBar.frame.width/5, tabBar.frame.height))

        // Uses the original colors for your images, so they aren't not rendered as grey automatically.
        for item in self.tabBar.items as! [UITabBarItem] {
          if let image = item.image {
            item.image = image.imageWithRenderingMode(.AlwaysOriginal)
          }
        }
      }
      ...
    }

And you will want to extend the UIImage class to make the plain colored image with the size you need:

extension UIImage {
  func makeImageWithColorAndSize(color: UIColor, size: CGSize) -> UIImage {
    UIGraphicsBeginImageContextWithOptions(size, false, 0)
    color.setFill()
    UIRectFill(CGRectMake(0, 0, size.width, size.height))
    var image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    return image
  }
}