I'm not sure if I understand your question correctly, and I'm only a beginner with Java 3D.  I'm assuming that your four sided 2D object is something like a four-sided plane like you would get by calling QuadArray.

Something like this:

     QuadArray plane = new QuadArray(4, GeometryArray.COORDINATES
                                     | GeometryArray.COLOR_3);

   //set coordinates  
     Point3f p = new Point3f(-1.0f,  1.0f,  0.0f);
     plane.setCoordinate(0, p);
     p.set(-1.0f, -1.0f,  0.0f);
     plane.setCoordinate(1, p);
     p.set(1.0f, -1.0f,  0.0f);
     plane.setCoordinate(2, p);
     p.set(1.0f,  1.0f,  0.0f);
     plane.setCoordinate(3, p);

   //set color
     Color3f red = new Color3f(1.0f,0.0f,0.0f);
     plane.setColor(0,red);
     plane.setColor(1,red);
     plane.setColor(2,red);
     plane.setColor(3,red);

Then, when you create the Appearance() object, set CULL_NONE in the PolygonAttributes like this:

     Appearance appear = new Appearance();
     PolygonAttributes polyAttrib = new PolygonAttributes();
     polyAttrib.setCullFace(PolygonAttributes.CULL_NONE);
     appear.setPolygonAttributes(polyAttrib);

     Shape3D planeObj  = new Shape3D(plane, appear);

When I ran this (in a rotating TransformGroup)I get a red square on both sides.  But if I set CULL_BACK or CULL_FRONT instead of CULL_NONE, I get a red square on one side, black on the other.  I hope this helps.