Was pasting the screenshots into Paint Shop Pro (a Photoshop clone) at various levels of zoom on the texture in question. Also, I realize that in using viz.screenshot(), certain areas of the image may be RGB-distorted due to the fact that JPEGs are a lossy format. That doesn't seem to be the issue, though.
Well, having tried this on another computer, the RGB values look fine. Neither computer has any external gamma correction or any such thing applied; the only differences that jump out to me is that the computer that worked had Vizard 1.098 and a Quadro2 video card; the one that doesn't has Vizard 1.097 and a Matrox G400.
I only care that the RGBs look fine on our rendering computer-- assume this is the case, unless you hear otherwise
Again, thanks for your help.