title: Python PIL: Create indexed color image with transparent background
body: <p>I wonder how I could create a image with a transparent background and only 2 indexed colours (red and blue) to minimise the file size?</p>
  
  <p>More specifically I have two black and white images that I want to convert, one to transparent and blue and the other to transparent and red. Then I want to merge those 2 images. I could do that with a regular RGBA image, but I really want the colour to be indexed to minimise the file size.</p>
  
  <p>Ideally with PIL, but other Python library could also work.</p>