CentOS there is no
DefaultDepth line in
/etc/X11/xorg.conf file to set the default color depth (although you can add this line to it). How does X determines which color depth to use?
It depends on the driver loaded, and defaults to 8-bit. From the
xorg.conf(5) man page (emphasis mine):
specifies which color depth the server should use by default. The -depth command line option can be used to override this. If neither is specified, the default depth is driver-specific, but in most cases is 8.