Fix Halo 2 Anniversary Darkness

First, You'll want to navigate yourself to the video settings. Here, make sure Halo 2 is selected under Game Setting. I know, there should be a difference between halo 2 and halo 2a, but here we are.

Scroll down slightly and you'll see the option under Video Settings called Gamma. Select it.
Change your Gamma settings from the original 5.0 up to 6.5. After that, your game should look a lot less dark and way more manageable!