title: set color of grouped stacked column in highcharts
body: <p>I have a requirement to set the color of stack column chart in relation with previous stack. 
  Consider this fiddle. <a href="http://jsfiddle.net/0n7g4a1e/" rel="noreferrer">http://jsfiddle.net/0n7g4a1e/</a></p>
  
  <pre><code>series: [{
          name: 'John',
          data: [5, 3, 4, 7, 2],
          stack: 'Actual'
      }, {
          name: 'John',
          data: [3, 4, 4, 2, 5],
          stack: 'Budget'
      }]
  </code></pre>
  
  <p>If Jane - Actual is light green, Jane - Budget should be dark green. This should be for all categories, Apple through bananas. 
  similarly if John - Acutal is light blue, John - budget should be dark blue. </p>
  
  <p>How can this be done? I dont want to specify each point color, i am fine with automatic color that gets selected, just that the bugdet series should have a shade darker than the actual.</p>
  
  <p>Thanks.</p>