title: How the color of attributed text inherits the parent color?
body: <p>For example I have to put a String "Hello, world!" in the <code>UIButton</code>.</p>
  
  <p>And "Hello," and "world!" should be different colors.</p>
  
  <p>"Hello," has the default color of <code>UIButton</code>, and "world!" has the customized color by <code>NSMutableAttributedString</code>.</p>
  
  <p>I explain again with code and result.</p>
  
  <pre><code>// Using a library `SwiftyAttributes`(https://github.com/eddiekaiger/SwiftyAttributes)
  button.attributedText = "Hello, ".withJust() + "world!".withTextColor(.red)
  
  button.setTitleColor(.black, for: .normal)
  </code></pre>
  
  <p><strong>I want:</strong> Hello,(black) world!(red)</p>
  
  <pre><code>button.setTitleColor(.red, for: .normal)
  </code></pre>
  
  <p><strong>I want:</strong> Hello,(red) world!(red)</p>
  
  <pre><code>button.setTitleColor(.blue, for: .normal)
  </code></pre>
  
  <p><strong>I want:</strong> Hello,(blue) world!(red)</p>
  
  <p><strong>But always the result is:</strong> Hello,(black) world!(red)</p>
  
  <p>...</p>
  
  <p>Maybe I think the priority of <code>NSMutableAttributedString</code> default color is higher than <code>UIButton</code> default color.</p>
  
  <p>But can I see the result I want?</p>