Controlling colors when displaying UnionSets

How can one control the display color rendered on the screen from a UnionSet? I have something like

   // construct river system
    Gridded2DSet[] river_system = {river};
    UnionSet rivers = new UnionSet(earth, river_system);

    // create a DataReference for river system
    final DataReference rivers_ref = new DataReferenceImpl("rivers");

    // create a Display
    DisplayImpl display = new DisplayImplJ2D("image display");

    // map earth coordinates to display coordinates

    ScalarMap longMap = new ScalarMap(RealType.Longitude, Display.XAxis);
    ScalarMap latMap = new ScalarMap(RealType.Latitude, Display.YAxis);

    // link the Display to rivers_ref

When I have run this code in the past, the river system has been drawn in white: today, it draws in black. Since I had always intended to grab control of the colors anyway, this seems like a good time.

Normally, I would hunt for the control on the relevant ScalarMap, which I would expect to look something like

        ScalarMap map = new ScalarMap( tempType, Display.RGB);

However, there isn't any mapping of the display type here: just the definitions of the axis.

Which brings me to a deeper question: since the above code only defines mappings for the XAxis and YAxis, why does anything appear at all?

Thanks in advance,