title: Opencv: Jetmap or colormap to grayscale, reverse applyColorMap()
body: <p>To convert to colormap, I do</p>
  
  <pre><code>import cv2
  im = cv2.imread('test.jpg', cv2.IMREAD_GRAYSCALE)
  im_color = cv2.applyColorMap(im, cv2.COLORMAP_JET)
  cv2.imwrite('colormap.jpg', im_color)
  </code></pre>
  
  <p>Then,</p>
  
  <pre><code>cv2.imread('colormap.jpg')
  # ??? What should I do here?
  </code></pre>
  
  <p>Obviously, reading it in grayscale (with <code>, 0</code>) wouldn't magically give us the grayscale, so how do I do it?</p>