Not sure exactly the effect you are after, but since no-one else has replied, maybe this will help get you started...
The Black and White effect can be used to make clips B&W throughout
Animating the Saturation parameter in HLS(Color Balance) allows color to be faded in/out/adjusted.
A second copy of the video in sync on V2 or higher with track matte transparency applied allows the area defined in a black and white mask in the next higher track to show only the mask area of the topmost video track. If that is in B&W and the main video is in color, the effect is a partial B&W area with the rest in color. The mask can be animated using Motion to alter the position/size/shape over time if required.