In trying to answer my own question as to what material to use if two are specified in the instance geometry in Collada 1.4.1 visual scene (please see my previous question: Duplicate Materials - what do I draw?) I noticed that at least one other author reading my DAE file into his viewer shows the same problem as I have:[attachment=2:3fc4fjgj]glc-player.jpg[/attachment:3fc4fjgj]. The roofs in black are the questionable material types here. (Please see: http://www.glc-player.net for a very good viewer of Collada files from which I produced this screen shot).
This image is remarkably similar to my much less sophisticated viewer:-[attachment=1:3fc4fjgj]IMG_0038.PNG[/attachment:3fc4fjgj]
However the Apple iMAC Collada viewer show the view I am expecting see:-[attachment=0:3fc4fjgj]mac_preview_version.png[/attachment:3fc4fjgj]

So my question is:- What does Google SketchUp, Google Earth and Apple iMac Collada viewer software know that me and at least one other developer doesn't? I would be most grateful to anyone letting me know the answer to this riddle .

Best wishes
Gordon Pagan