LOGITECH GAMING SOFTWARE
Since the G600 comes without a software CD you should first download the latest version of the gaming software from the official Logitech support page.
The Logitech Game Software has a very user friendly and colorful GUI. From within the game software you can switch between the onboard profiles or the ones stored on the computer, map the 20 available buttons (both normal/G-Shift modes), configure game profiles (each game can have up to three different ones), set macro commands, set the DPI sensitivity levels (up to 5 different ones), set the polling rate, enable/disable advanced functions and finally set the button backlight color from one of the 16 million available (steady color, pulse, cycle).
You can also adjust several less important settings and check the current firmware version (although according to the software it's not upgradable) from the settings tab.
Only the 12 thumb buttons feature backlight and you can pick from a total of 16 million colors available.