I spent a full hour playing around with the settings yesterday.
If you want to try it do what I did, start a solo map, then pause the game (pause key) and open the console.
You can adjust the brightness, gamma and contrast in real time without having to enter/exit the menu.
I belive the default settings were something like this:
brightness 0.9
gamma 0.9
contrast 0.8
Just change the values and see for yourself.
I've noticed that if you increase the Gamma and the Contrast a lot you get vibrant colors, which looks cool and sometimes really pretty but it will also ruin your eyes (mines at least).
After a while I decided to leave the gamma on 0.9 and simply play around with the contrast (brightness should be similar to the contrast for the best effect). Im getting a sort of grey-ish tone (sort of like the default settings) but at least my eyes dont bleed each time i turn around a corner and encounter a bright light.
I wish there was a separate command to adjust color saturation, that would make things look way better, but I believe this is a post processing effect not supported by the engine (citation needed).