change your XF86config in /etc/X11 the screen section is what you should look at add a screen with 16 bit
there are a couple of readmes on the net it should look something like this:
# ----------------------------------------------------------
# SCREEN section
# ----------------------------------------------------------
# This section defines the available resulutions and depths.
# If you disable CRT2 (by setting ForceCRT2Type to "NONE"),
# you can use higher resolutions than the LCD panel supports.
Section "Screen"
Identifier "screen1"
Device "SiS 630"
Monitor "Generic|Generic LCD Panel"
DefaultDepth 16
SubSection "Display"
Depth 16
Modes "1024x768"
EndSubSection
SubSection "Display"
Depth 8
Modes "1024x768"
EndSubSection
SubSection "Display"
Depth 24
Modes "1024x768"
EndSubSection
EndSection
you have to change display and graphic card to fit your needs