code: def add(colour)
           if colour.instance_of?(Colour)
              @colours.push(colour) if @colours.index(colour).nil?
           end
           self
        end
docs: This method adds a new colour to a ColourTable object. If the colour
   already exists within the table or is not a Colour object then this
   method does nothing.
  
   ==== Parameters
   colour::  The colour to be added to the table.