I have a Linux application "figdisp" developed long ago for use on a PseudoColor (depth=8) display, so my former dual-boot setup had two monitors configured as separate screens (not Xinerama), where screen :0.0 was TrueColor and screen :0.1 was PseudoColor ... when starting "figdisp", I would direct it to the :0.1 display screen.
Now on Linux under VirtualBox, I'm using the Linux vncserver to create a virtual PseudoColor display:
% vncserver :1 -name Figdisp -geometry 1912x1146 -depth 8 -cc 3
... that I can then view on my Windows 7 host's TrueColor display. When I run the "figdisp" application with -display 127.0.0.1:1 -etc, the application reports that it "Got 159 bitmap colors and 16 line-graphics colors in default color map" ... which is exactly as I expect (in the vncserver's display :1, I'm using mwm as a no-frills window manager, leaving 175 colors in the default color map unassigned until the "figdisp" application starts). Initially, figdisp assigns greyscale values to the 159 bitmap colors and displays a "test pattern" with these greyscale values running diagonally ... here is the Figdisp vncserver 127.0.0.1:1 display as viewed using Linux vncviewer on the local Linux VM:

(Click on the image above to view the native 1920x1200 pixel wuxga screen capture)
But when I close the Linux vncviewer and reconnect to the vncserver display using UltraVNC-viewer from the Win7 host computer, instead of rendering smoothly varying shades of grey, the UltraVNC-viewer shows distinct diagonal bands. I count only 32 levels of grey in the greyscale image rendered by UltraVNC-viewer (not the 159 bitmap colors that "figdisp" reserved in the default color map and is using to render the image).

(Click on the image above to view the native 1920x1200 pixel wuxga screen capture)
Is there a setting in UltraVNC-viewer on Windows that I can change that would force it to use all of the colors in the PseudoColor display's default color map when rendering this greyscale image ? The advantage for me of using a VNC-viewer running on the Windows host is that, if I have a second monitor, it's trivial to drag the Windows VNC-viewer application over to the second monitor, so that I can see both the Linux VM session (TrueColor) and the "Figdisp" (PseudoColor = vncserver :1 -depth 8 -cc 3) display simultaneously on adjacent screens, thus duplicating the functionality my previous dual-boot arrangement had when booted to Linux.
I've tried many of the "Format and Encoding" options available in UltraVNC viewer, but the banding of greyscale levels was similar in all those that I tried (including Hextile -- used by my Linux vncviewer -- and Raw, Ultra, and u2...), with and without checkmarks against Zip/Tight compression and Jpeg (Tight) Quality.
Before hunting-for and trying-out other Windows VNC-viewer applications, I thought I would post here to see if I just haven't found the right setting(s) for the Windows UltraVNC-viewer.
Thanks in advance,
--Jim