title: Add custom colors to Xcode attributes inspector
body: <p>I have a Swift class that extends UIColor and has functions like:</p>
  
  <pre><code>class func buttonShadowColor() -&gt; UIColor
  {
      return UIColor(red:0.74, green:0.74, blue:0.74, alpha:1)
  }
  </code></pre>
  
  <p>Is there any way to add these colors to the "text color" (or any dropdown) in the attributes inspector so they can be used app wide from the storyboard?</p>
  
  <p><img src="https://i.stack.imgur.com/KruyU.png" alt="enter image description here"></p>