title: How to get channel of any colour from a photo?
body: <p>RGB consists of three channels - red, green, blue. How do I get lets say brown channel out of rgb?</p>
  
  <p>I need it to be able to recognize brown (rusted) objects in a photo, so it would be nice to convert the photo into a "brown channel".</p>
  
  <p>EDIT:</p>
  
  <p>Here is the photo from which the brown colour should be recognized:</p>
  
  <p><a href="http://dl.dropbox.com/u/4571/section.jpg" rel="nofollow">http://dl.dropbox.com/u/4571/section.jpg</a></p>