RealType with Multiple Mappings

My project is a combination of DisplayTest 21 (netcdf file) and 27
(track pointer).  The scalar maps look like this:

    final ScalarMap map2lat
      new ScalarMap((RealType) dtype.getComponent(0), Display.XAxis);
    display.addMap(map2lat);
    final ScalarMap map2lon
      new ScalarMap((RealType) dtype.getComponent(1), Display.YAxis);
    display.addMap(map2lon);
    final ScalarMap map2doc
      new ScalarMap((RealType) rntype, Display.ZAxis);
    display.addMap(map2doc);
    display.addMap(new ScalarMap((RealType) rntype,Display.Green));

Later, for tracking the pointer I use

    RealTuple direct_low = new RealTuple(new Real[]
      {new Real((RealType) dtype.getComponent(0), range1lat[0]),
       new Real((RealType) dtype.getComponent(1), range1lon[0]),
       new Real((RealType) rntype, range1doc[0])});

This fails with

  DIRECTMANIPULATIONRENDER_???.DOTTRANSFORM REALTYPE WITH MULTIPLE MAPPINGS

Now if I comment out

  display.addMap(new ScalarMap((RealType) rntype,Display.Green));

the yellow dot appears and I can track the pointer.  However, the
surface is now shades of silver/gray rather than shades of purple.  How
can I have both features: 1) track the pointer by grabbing the yellow
dot and moving it around; and 2) shade the surface in something other
than silver/gray?

Thanks in advance for any advice.