The enumeration list of standardized names developed for the
              International Centre for Diffraction Data.
              The colour of a crystal is given by the combination of
              _exptl_crystal.colour_modifier with
              _exptl_crystal.colour_primary, as in 'dark-green' or
              'bluish-violet', if necessary combined with
              _exptl_crystal.colour_lustre, as in 'metallic-green'.