title: JXL Attempt to modify a referenced format while setting background color
body: <p>While trying to execute the below code:</p>
  
  <pre><code>Colour selectedColour = Colour.WHITE;
  myWritableCellFormat.setBackground(selectedColour);
  </code></pre>
  
  <p>I am getting this Exception:  </p>
  
  <pre><code>jxl.write.biff.JxlWriteException: Attempt to modify a referenced format
  </code></pre>
  
  <p>Why is this? For reference I've used the same object before these 2 lines to write a cell with another color. Is this of any importance?</p>