def change_fill(rgb = 'ffffff')
      validate_worksheet
      Color.validate_color(rgb)
      self.style_index = workbook.modify_fill(self.style_index, rgb)
    end
def change_fill(rgb = 'ffffff')
      validate_worksheet
      Color.validate_color(rgb)
      self.style_index = workbook.modify_fill(self.style_index, rgb)
    end
Changes fill color of cell