I just saw this the other on tv, but there was a shot of a man walking into a room and the shot froze and the everything in the back ground turned black and white while the man was still in his full color while frozen.
Do they achieve this by having two of the same videos playing (but one video has the man cut out and the back ground is black and white) and at the moment you want that look you just freeze the shot?
You take the clip and slow it down with a behavior or keyframes. Then you duplicate the clip. Use the mask tool on the top clip to create a mask around the person. Then on the bottom clip add a desaturate filter to make it turn from color to black and white. Hope this helps and best of luck.