For some reason this has only started over the past week or two, so it could very well be a driver issue.
Grunts and brutes alike switch from being normally coloured to being bright shiny white with barely any distinguishing features other than the redness of the grunt's mouth/claws, and tend to remain in this state for 99% of the time. This makes them visible at great distances, as they seem unaffected by the game's lighting/shadow, which obviously sucks.
Any workarounds for this save for downgrading my GPU drivers? I recently installed updated drivers and the problem persisted.
Cheers
EDIT 7th September 2013:
I think this has been solved. If you edit the sanity level in your save file (for instance, if you want to cheat) this problem can occur. Presumably this is because of the way in which looking at monsters affects your sanity and causes visual effects for the player. If the sanity value has been tweaked manually it can cause the enemies to become pure white, as you can see in the screenshots
below.
The specifics of this are still a mystery to me, but I have experimented and narrowed it down to the sanity level value in the save file. During the discussion below I stated that I played the game with and without cheating and that the problem occurred regardless, but when I played without cheating I merely edited the values back to their original states; this was obviously not good enough. Editing the value in
any way can potentially cause these issues.