xdm using 8bpp: how?

I remember reading here not long ago about how to setup multiple xdm sessions with different color depths. But in a fit of tidyness I erased the message and can't find the answer in any of the manpages or informational docs that came with X. I would like to start up my single session with

Re: xdm using 8bpp: how?

I remember reading here not long ago about how to setup multiple xdm sessions with different color depths. But in a fit of tidyness I erased the message and can't find the answer in any of the manpages or informational docs that came with X. I would like to start up my single session with