Recently I packaged ddccontrol for Fedora. It is nifty tool that allows you to change settings of your external monitor (brightness, backlight, RGB, ...) from your desktop or CLI. It uses DDC/CI protocol thus this functionality needs to be supported by your monitor and graphics card. Personally, I tested it with Intel video cards (i915) and recent Nvidia video cards (with binary driver) and it worked correctly. It should also work on others. There is a list of supported HW in upstream documentation, but the list isn't exhaustive, thus give it a try even if your HW is not listed there. In case it doesn't work don't forget to try all other video connectors (on both monitor and video card) because this functionality is sometimes supported only on a subset of installed connectors. You will also need i2c_dev kernel module, because this functionality requires I2C.
For quickstart there is GTK GUI application called gddccontrol (package ddccontrol-gtk). It allows you to create/manage/switch profiles, thus it is possible to have different contrast/brightness settings for day/night or gaming/work. For all controls to work correctly your monitor needs to be in ddccontrol database otherwise there will be only generic VESA controls. In this case you can create the profile for your monitor yourself or at least send report to upstream (ddccontrol-users AT lists.sourceforge.net) containing the output from the following command:
LANG= LC_ALL= ddccontrol -p -c -d
For more details and instructions how to use the CLI tool see upstream documentation.
