How to Enable 30-bit color on Linux
Quadro cards based on G80 (G=Generation) and higher are required. You need to modify the "Screen" section of your configuration file (usually xorg.conf) by replacing 24 with 30 where it appears. For example, look at this xorg.conf "Screen" section:
You need to replace both instances of 24 with 30, save the changes, and restart the X server.
There is more information in the README file that is included with the NVIDIA Linux Display Driver in Chapter 31, "Configuring Depth 30 Displays."