The XF86Config-4 file contains the option named DefaultColorDepth, while
this option is called DefaultDepth in XFree86 4.0. Despite this,
DefaultColorDepth still seems to be recognized as an alias name, but this
is undocumented and might change tomorrow...
Furthermore, the default color depth is included at the end of the screen
section, unlike the other settings. I would suggest to put it in the
beginning of the screen section, to avoid that people (like me... :-)) add
their own entry above and don't see that another entry appears at the end
of the section.
This is the same as bug # 14852 for Xconfigurator.
Anaconda has always written "DefaultDepth" at the top of the section.