title: change the color of image in matlab
body: <p>in matlab I change the color of some pixels of an image to black but the color changes to blue.
  I can't understand the problem.My code is below. could any one help me?</p>
  
  <pre><code>img = imread('test.png');
  for i = 1 : 200
    for j = 1: 640
      img(i,j) = 0;
    end 
  end
  </code></pre>