For the MBA 3,1 and 3,2; the installation howto for Narwhal has instructions to smooth color gradients by adding to "/etc/gdm/Init/Default" the following
Desktops like 11.10 (Oneric Ocelot) running LigthDM can't use those instructions, though. How could this be done for this DM?
/usr/bin/nvidia-settings -a [gpu:0]/Dithering[DFP-2]=1
/usr/bin/nvidia-settings -a [gpu:0]/DitheringDepth[DFP-2]=1
PS: Reverting to gdm, for some reason, makes graphics slow (but smooth because of the fix mentioned above)