Interior "bug" ?

I'm trying to get this darn daylight cycle working properly. I need to change the vertex colors of all vertecies that are visible to the outside. There's a SurfaceOutsideVisible surfaceMask that apparently does this. The debug render using this works fine, and if I replace the current render with the debug render, that works fine too.

All I'm adding is:

if (rSurface.surfaceFlags & SurfaceOutsideVisible)
   glColor3f(DaylightCycle::getInteriorColor());
else
   glColor3f(1,1,1);

What I get is spontaneous effects, where some light correctly, and some light the opposite of what they should.

As I understand it, the interiors are rendered surface by surface. So, it would be possible for a surface test to see if it's visible from the outside and just modulate it by a precalculated color. But, apparently, that's not so.

The question is:
Am I just missing some mundane (SP?) detail, or do I have to go about making a color array to add into the existing vertex arrays in order to modulate vertices properly?

This is about the 3rd time I've asked, and I've researched this to death, but I can't get anywhere. So someone please respond so I get get this thing done and off my mind!