title: How replace transparent with a color in pillow
body: <p>I need to replace the transparency layer of a png image with a color white. I tried this</p>
  
  <pre><code>from PIL import Image
  image = Image.open('test.png')
  new_image = image.convert('RGB', colors=255)
  new_image.save('test.jpg', quality=75)
  </code></pre>
  
  <p>but the transparency layer turned black. Anyone can help me?</p>