XDM setup, how to change default color depth

[Chris R. Martin]

How can I convince xdm that I want to run at a 32-bit color depth by
default rather than 8 bpp?  Do I need to 'hack up' my XF86Config file so
that 32 bpp is the only one availible, or can I somehow pass a parameter to
the X server when it is started up?

Thanks,
Chris