def column_shading_colour(index, colour)
         self.each do |row|
            cell = row[index]
            cell.shading_colour = colour if cell != nil
         end
      end
def column_shading_colour(index, colour)
         self.each do |row|
            cell = row[index]
            cell.shading_colour = colour if cell != nil
         end
      end
This method assigns a shading colour to a specified column within a
 TableNode object.

 ==== Parameters
 index::   The offset from the first column of the column to have shading
           applied to it.
 colour::  A reference to a Colour object representing the shading colour
           to be used. Set to nil to clear shading.