You can start X manually with different color depths from runlevel 3 with 
the following command:

startx -- -depth 24

where the number after "depth" can be 8, 16, 24, 32...

more information can be found with:

man startx

...

Other fun stuff when troubleshooting X sessions:

You can also start more than one X session without quitting your current
one if you wish to experiment:

startx -- -depth 16 :1

where :1 can also be :2, :3, :4... and flipping between the different 
Xsessions can be done with ctrl-alt-F7, ctrl-alt-F8, etc...  You can even 
launch applications on the other X sessions, even on other computers on 
the internet with the DISPLAY environment variable:

export DISPLAY=attaway.net:1

if I ran the xhost command within my X to allow permissions for your host 
to run on my computer.

Be warned there is a bug in at least one older version of redhat where the
environment will cause the system to crash if X is started from an Xterm.

On Tue, 3 Sep 2002, Aldis A. Tuck wrote:

> Greetings,
> I installed MDK 8.2(kernel 2.4.18-6), I had video but
> it wasnt accurate.  In mandrake control center it said
> it was at resaolution 1280X something, but the screen
> size was clearly 800X600.  My color depth was
> 24billion
> I change it to 16Million and then it told me to
> reboot.
> Now it will not load into X anymore, it goes to
> runlevel
> 3.  I tried to run XFree86 --h, but I dont know what
> to
> do to fix it, I think I just need to change the color
> depth back but I dont know how. when I do the help
> thing
> it scrolls all the stuff I probably need to see off
> the
> screen.  Could someone please instruct me on how to
> change
> my color depth?
> 
> Kind Regards,
> 
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