I took a bit of time this morning to experiment further with this problem.  By now I am convinced there is nothing wrong with the mesh, the way I exported it or the way I imported it.  The problem, I thought, had to be in the MAT itself.

So I conducted a few experiments.  First I loaded up a new scene, imported the wall again and applied the Marble Mat.

Next I went in and disconnected the granite note and plugged the other nodes used a simple color math node to connect the other nodes and plug them back into the output.  Rendered... no color issue.

So that told the problem was with the granite node itself.

Next I made a default granite node and plugged it in the way the orginal had been but left its settings default.  Rendered... no issue.

So it wasn't the granite per se but a setting.

Next I tried plugging the original granite node back in (basically reset the MAT) and then adjusted the Scaling in the granite node from 0.25 to 0.5 and rendered... color issue partly gone but still present.  Ah ha!  Now we're getting somewhere.

I upped Scaling to 0.75 and the color shift issue disappeared completely.

So there's the crux of the problem.  Now my next question is... why is scaling it down to 0.25 (the default in the MAT) causing an issue.  What does scaling in the granite node do and how is that interacting with my mesh to create this problem?  Any node experts out there.   Paging Baginsbill?