is Xvfb colorblind?

I’m trying to run a graphical application in the background of a headless server for scientific work. But when I run Xvfb with basic values, it croaks if I attempt to access a shared or private colormap. What’s up with that?!? Works perfectly if I ship it to a remote terminal, but that’s not an option for what I need.
Is there an argument that specifies the color depth or something? Running SLES 9/10, but don’t think that should really matter.

OK, I posted without enough info. I requested 8 bit color (WxHx8), but if I shoot some simple windows at my virtual display and grab the results with an xwd, every pixel is either black or white. If I try to display a color image, it either gives an X_StoreColors failure, or just plots solid black.

The plot thickens.
When I run the same procesdure on a machine with a graphics head, it works perfectly.
Here’s my procedure:

> Xvfb :3 -ac -screen 1 1024x768x8 +bs -fbdir /tmp &
> setenv DISPLAY :3
> xv pretty_pix.jpg &
> xwd -display :3 -root -out window_1.xwd

So on one machine it gives me the expected desktop image. On the headless machine, I get an image of a solid black window on an empty desktop. Or, if I try another application,using a private colormap, it gives an error:

X Error of failed request: BadAccess (attempt to access private resource denied)
Major opcode of failed request: 89 (X_StoreColors)
Serial number of failed request: 2061
Current serial number in output stream: 3227

Any takers?