Query: Set screen color depth on a per-user basis via GDM?

Students in my lab run various apps on a couple of central SGI IRIX 
and HP-UX systems, which each then display X graphical output 
simultaneously on multiple remote Redat 7 linux boxes.  The output is
a Geographical Information System 2D map-like display.

One of these server apps only generates the remote graphics
correctly if the receiving linux boxes are in an 8-bit color depth.
If set to higher depths they either generate fatal colormap run-time 
errors or just generate screwed-up and non-usable maps.  We only have 
binaries of the server apps, so have to work with them as they are.

However, on other occasions, the students want the same linux screens 
set in the highest color depth possible to use the other server app, or
when using GIMP, StarOffice, etc.

We did consider using CTRL-ALT-PLUS/MINUS to cycle through
preconfigured system-wide color depths, but this proved too unwieldy and
confusing, especially as the students are not very linux literate.

One brute-force solution seemed to me to set different color depths
for different user accounts on each box, so all they have to remember is
which of two usernames to use.

If we were using startx from the command line, this wouldn't be too
difficult.  However we're using GDM for graphical logins, because of the 
general user expertise level as mentioned above.  What I can't see how to 
do is to set-up GDM to start/restart each X session at different colour 
depths for different users.  Setting up differet window-managers, etc 
can all be done per-user, so what about screen depth?

Does anyone know if it's possible and how I should go about setting
it up?

Or is there a more elegant way of doing this?

Jonathan Searle
Cranfield University

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