Thanks

spongylover123 wrote:
Env_fogcontroller or something like that

You are right.

You can give an output to the player to give him a special fog controller.
For example:

OnTrigger
!activator
setfogcontroller
fog1

The last line is the name of the fog controller you want to give the player.

If you watch the instance coop\coop_lighting_ents.vmf you can find also 2 more !players.
!player -> normal player, singleplayer.
!player_orange -> orange player
!player_blue -> blue player
So you could also give the blue player another fog than the orange player.
For example:

OnTrigger
!player_orange
setfogcontroller
fog_orange

If this gets triggered, the orange player will have another fog than the blue player.

Blue fog in Room A
Grey fog(or none) in Room B

Just create triggers between chambers and set outputs wich activates other colored fog.

And if you have huge room, you can't put fog for one side, it must be whole level at once, but it can be disabled, enabled or changed.