With the XFree86 servers on the i386 (and other architectures that
also use the XF86_* servers), you can set the default color depth with
the XF86Setup program.
You can also directly edit /etc/XF86Config qnd add a
`DefaultColorDepth' entry in the "Screen" section corresponding to
your server. See the XF86Config(5) manual page for details.
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Matthieu