An object of class "colourmap" is effectively a function that
  maps its inputs (numbers or factor levels) to colour values.
  
The command colouroutputs(x) extracts the colour values
  in the colour map x.
The assignment colouroutputs(x) <- value replaces the colour
  values in the colour map x by the entries in 
  value. The replacement vector value should have the
  same length as colouroutputs(x), and its entries should be
  interpretable as colours.
To change only some of the colour values in a colour map,
  it may be easier to use tweak.colourmap.