There’s an awesome utility called F.lux which automatically tweaks your monitor colors throughout the day, to make your display easier on the eyes. The problem is, it doesn’t work with fullscreen games, by default.
Luckily, there is a cool utility called Color Clutch which was created as a way to get around the fact that Windows desktop color calibration doesn’t apply to DirectX fullscreen mode. It accomplishes this using function hooking, so it won’t work for all games (games with strict anti-cheat detection will get a false positive on it).
To get this working, just download Color Clutch from the website above. Create a batch file with contents like this (you will need to use the correct paths):
inject.exe "D:\cclutch\cclutch_ix.dll" patch "C:\Program Files (x86)\Guild Wars 2\gw2.exe"
Now, just run that batch file when you want to launch Guild Wars 2.
If the game you want to use F.lux with is using a different version of DirectX, you’ll need to modify the batch file to point to a different version of ccluch_*.dll – easy enough.