title: Image not saving after updating pixel color in PIL
body: <p>I've generated barcode images in PNG format using HuBarcode, and have modified them to add a border of <code>rgb(150, 150, 150)</code> in hopes of accessing the pixels of that color to change them. I can access the pixel and get confirmation via <code>print</code> that the color is being changed, but when I open the image file, nothing has changed. The pixels are still <code>rgb(150, 150, 150)</code>. Here's a code snippet I'm using. I can add more of my code if it will be helpful:</p>
  
  <pre><code>def add_colored_border(self, barcode):
    img = Image.open(barcode)
    img = img.convert('RGB')
    px = img.load()
    for x in range(img.size[0]):
      for y in range(img.size[1]):
        pixel = px[x, y]
        if pixel == (150, 150, 150):  
          pixel = (0, 0, 255)
    img.save('testing.png')
  </code></pre>