title: Adding colour overlay to greyscale image
body: <p>I'm wanting to add pre-generated heatmaps over photographs. The colours in the images aren't important and to make the heatmap colours stand out I'm making the images greyscale. I've done this using </p>
  
  <pre class="lang-py prettyprint-override"><code>    grey = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
  </code></pre>
  
  <p>However the greyscale image now has one fewer dimensions compared to the heatmap (which is BRG). How can I overlay the heatmap on top of the grey image?</p>