title: How to add initializers in extensions to existing UIKit classes such as UIColor?
body: <p>The Swift documentation says that adding initializers in an extension is possible, and the example in the document is about adding an initializer to a <em>struct</em>. Xcode doesn't recognize <code>UIColor</code>'s designated initializer in my convenience initializer: </p>
  
  <pre><code>extension UIColor {
    convenience init(rawValue red: CGFloat, green g: CGFloat, blue b: CGFloat, alpha a: CGFloat) {
  
      // Can not find out the designated initializer here
      self.init()
  
    }
  }
  </code></pre>
  
  <p>Any solutions?</p>