title: OpenPyXL: AttributeError when trying to set fill
body: <pre><code>    ws.cell(row=greenBegin+1, column=3).style.fill.fill_type = 'solid'
  
      ws.cell(row=greenBegin+1, column=3).style.fill.start_color.index = '0000FF'
  
      ws.cell(row=greenBegin+1, column=3).style.fill.end_color.index = '0000FF'
  </code></pre>
  
  <p>I'm attempting to set the fill of a cell to solid green using this code, but I always get "AttributeError: can't set attribute" when I run the code. I've tried different styling methods, but there isn't a whole lot of information on this library AFAIK which is bothersome to say the least as I'm using it for a work project.</p>