There's an issue with the current AMD drivers and 6xxx cards with GTAIV (no surprise there!)
If you get green or red flashes all over the screen when it starts raining, you just need to change the shader the game is using.
This may only be an issue with the Episodes from Liberty City expansion, but it might affect vanilla GTAIV as well.
If you have the Steam EFLC, go to C:\Program Files\Steam\steamapps\common\grand theft auto iv episodes from liberty city\EFLC\common\data and edit the visualSettings.dat file. (Different directory will apply for other games/delivery media).
and change it to
This also seems to fix textures not being rendered at all, leaving smeary black marks all over the screen, just like in GTAIV 1.0.0 with the AMD driver of the time.
All things Grand Theft Auto and its many bastard offspring