code: def column_shading_colour(index, colour)
           self.each do |row|
              cell = row[index]
              cell.shading_colour = colour if cell != nil
           end
        end
docs: 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.