In this videotutorial, BlenderDiplom shows how to achieve the 'Sin City' look (B&W video with some parts in color).
Gottfried Hofmann writes:
There are so many things you can do with Blender since the tracker has been added - for example applying the Sin City look to your footage using nothing but Blender. If you want to apply the Sin City look to your footage, Blender now has all the required tools at hand. This tutorial shows you how to do it using the compositor and the tracking- and masking-tools.