title: using bgr2gray in python are not modifing to gray
body: <p>I am importing an image in python using <code>opencv</code> and trying to change it to grayscale, but it won't change and it returns back in a hue like colors </p>
  
  <pre class="lang-py prettyprint-override"><code>def grayscale(img):
      img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
      return img
  
  img = grayscale(X_train[1000])
  plt.imshow(img)
  plt.axis("off")
  </code></pre>