title: What does the following colon (:) mean in MATLAB syntax?
body: <pre><code>a = imread('autumn.tif');
  a = double(a); 
  [row col dim] = size(a);
  red = a(:, :, 1);
  green = a(:, :, 2);
  blue = a(:, :, 3);
  </code></pre>
  
  <p>What does the colon <code>:</code> in the last three lines mean? (The above snippet is from "Image Processing" by Dhananjay Theckedath.)</p>