title: Change color of certain pixels in a UIImage
body: <p>For a given multi-color PNG <code>UIImage</code> (with transparency), what is the best/Swift-idiomatic way to:</p>
  
  <ol>
  <li>create a duplicate <code>UIImage</code></li>
  <li>find all black pixels in the copy and change them to red</li>
  <li>(return the modified copy)</li>
  </ol>
  
  <p>There are a few related questions on SO but I haven't been able to find something that works.</p>