title: Add a transparent coloured background to png with transparency
body: <p>I'm trying to do something very simple but somehow can't manage to do it.</p>
  
  <p>I have an PNG with transparent areas.</p>
  
  <p>How to add a background layer filled with some RGBA color such as <code>rgba(255,0,0,128)</code>?</p>
  
  <p>I'm using imagemagick version 6 in its command line form.</p>
  
  <p>This would be the PNG image for example:
  <a href="https://i.stack.imgur.com/SvifP.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/SvifP.png" alt="enter image description here"></a></p>