The effect gets applied to the whole frame, so the buildings are still saturated aswell as you see on your picture. Either you desaturate the texture files of the game or you work with 2 layers in your video software. Use 'Cookie Cutter' or mask the building on the upper layer and add the FX 'Black & White' with maximum value. You'll have to move that mask on every frame though. That might be faster than editing the textures. It's kinda lavishly either way.