Solution: man XF86Config
and look under the Screen section for
a little happy option called DefaultColorDepth :)
Here's my extract from my XF86Config:
Section "Screen"
Driver "accel"
Device "S3 864"
Monitor "OPTIQUEST 4000DC"
DefaultColorDepth 16
SubSection "Display"
Depth
16
Weight
565
Modes
"1152x864_16" "1024x768"
ViewPort
0 0
Virtual
1152 864
EndSubsection
SubSection "Display"
Depth
32
Modes
"640x480"
ViewPort
0 0
Virtual
800 600
EndSubsection
Subsection "Display"
Depth
8
Modes
"1152x900" "1152x864" "1024x768" "800x600" "640x480"
ViewPort
0 0
Virtual
1152 900
EndSubsection
EndSection
