Carriers is a multiplayer Half-Life 2 mod being created by the Shellshock Gaming development team. It's based around a modern-day zombie apocalypse, where players must work together to survive. But now the luck of the carriers has run out, and their immune systems are failing.
This is just to compare.
Is it a kind of filter?
It's colour correction, which alters the colour balance to give off a particular atmosphere.
I still don't know why it was so grey before...