title: How to replace a color with another using GraphicsMagick Nodejs
body: <p><br/>
  I have an image with black background. I want to change it to have white background. Is that possible to achieve using GraphicsMagick in Nodejs?
  <br/>
  I tried <code>gm("test.png").fill("white")</code> but this didn't work for me.<br/>
  I did some search and found there is a convert command but not sure there is equivalent function for it. <code>gm.convert</code> ends up giving error.</p>