Thanks everyone for your assistance. I think I understand now what was going on. Here is a short video of what I found for those who might have similar issues: http://screencast.com/t/lhMJWSSsay
In brief:
Reset the material used for the walls in the ROOM SPECIFICATION dialog to 'Default' Reset the material used for the walls in the WALL SPECIFICATION dialog to 'Default' Make sure the Wall Type is correct and is using the colour and material required.
One thing i didnt expect was that once a material is chosen for a Wall Type, there is no longer any link between the material and the wall material. So changing the material (mine was a custom one) has no affect on the wall. The 'Material Name' in the Wall Type Definition still says the name of my custom material. But it is NOT showing the changes to my custom material. You have to reselect the custom material for the changes to it to apply to the wall. This seems like a bug to me. Either it shouldn't show the 'Name' of the material because the material can change, or it should clear the name when the material changes or it should update the wall type material to reflect the changes to the custom material. My preference would be option 3.
agrogers's post in Are There Material 'styles' In Ca? was marked as the answer