How to change the color depth?

You can create a new xorg.conf by switching into a virtual virtual console (Ctrl + Alt + (F1-F6)) and running sudo service gdm stop.

Then run Xorg -configure (yes, it should be Xorg, not xorg). If you had an old xorg.conf file in /etc/X11/ you'd first back that up by doing sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.backup.

Then move your newly created xorg.conf to /etc/X11/ by running sudo mv xorg.conf.new /etc/X11/xorg.conf and restart gdm by running sudo service gdm start.

Then you can change the color depth in there by finding the appropriate section and changing/adding whatever's in there to DefaultDepth 16

Try doing this step, but before you get to the sudo service xdm start part, change the screen section (within the xorg.conf file) to something like:

Section "Screen"
Identifier    "Default Screen"
Monitor        "Configured Monitor"
Device        "Configured Video Device"
DefaultDepth    16
Modes        "1024x768"
EndSection