当在Xvfb而不是普通X下生成时，图像具有fubar调色板

When generating a png image file in Mathematica on a linux box running X, there's no problem. Doing the same on a linux machine without X but instead Xvfb the png generates fine but the colors are all screwed up. We tried changing the pixel depth from the default of 8, to no avail. Any other tweaks that might make Xvfb mimic normal X better, or is this more likely a Mathematica bug?When generating a png image file in Mathematica