I know this is an old thread but in the process of re-restoring my 67 5B NOR ragtop and wanting to return the underbody to the correct finish method, I have been studying this topic in detail for a long time and have also read the 'Process Overview' article many times. When I restored the car back in the mid 80's I scuff sanded and sprayed the under body semi gloss black for the clean look. I remember seeing yellow but I never kept record of how much.
Now I want to undo that. No matter where in the undercarriage I sand I uncover the following layers (going from top layer to bare metal).....top layer--> black (my paint from the 80's) --> butternut yellow (original body color and paint) --> red oxide primer layer--> what looks like a dark gray or black very thin layer (phosphate?) --> bare metal. There is no gray to be found underneath so far as reports suggest should be there and overspray over that.
What stumps me is the whole bottom has a layer of butternut yellow including deep inside the tunnel at the top...as if the whole bottom was sprayed. I figured the robotic sprayers would hit the bottom edges mainly and not so much up and into the tunnel top.
These pictures, especially the green example, prove to me that it is possible to have an almost underbody color the same as the top coat. It may sound anal but I am so relieved to see this example and know it can happen.