I think KDE runs it's own. But to reduce the color depth edit your /etc/X11/XF86Config (XF86Config-4 if you are using XFree4.x) and change the default color depth in there. The line should stick out like a sore thumb, if not, you could always run xf86config and change it that way.
There may be other, faster, easier ways, but that's probably how I would do it.
Cool
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