That seems to be correct - earlier in 12.04 dev you could edit greeter settings thru gsettings, now you obviously can't.
Originally Posted by NHclimber
So to mention - an alternate to the cli switching to lightdm user, ect would be to create an override file & compile.
Ex. with 3 of the settings though don't see much purpose to the color one
sudo gedit /usr/share/glib-2.0/schemas/50_unity-greeter.gschema.override
play-ready-sound = false
background-color = "#000000"
draw-grid = false
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/