Once you find game.cfg, open it and delete the sys_spec line. Now your custom settings will stick in multiplayer, but if you ever use the in-game graphics menu, the game.cfg file will be rewritten with the sys_spec line intact. Change the properties of game.cfg so the file is read-only to keep that from happening.
Disabling colorgrading makes it colorful (it removes the large amount of blue with it turned On. The blue color, predominating in the majority of the maps, makes the 'immersion in darkness' stronger. For those having everything white it makes the brightness brighter). Colorgrading is somehow in conflict with the brightness level.
[FIXING* THE PROBLEM]
It seems Brightness is at default settings, differing from what it should be. Some people with the 'dark screen of death' have it set to 0, while there are people experiencing extremely bright in-game. Those have it set to 1. Meanwhile 0.5 would be the correct value.
Type at the console or add it at your config (.cfg) file (C:\Users\YOUR NAME\Saved Games\Crysis2)
r_Brightness = AMOUNT OF BRIGHTNESS SET ON YOUR SCREEN (ex:0.55)
r_colorgrading = 0
r_ssao = 0
r_ssdo = 0
If you set a different level of brightness, then you'll have to change it back to your actual brightness level manually every time you quit the game. Go to control panel, enter your vga control panel and change it back to the amount you've set before doing it.