How is the Sin City effect done, where only particular color in the video is moticeable and everything else is Black & White. Also can I do this with a keyed out background, similar to this
For some reason I can't see the link. Time to reboot the computer.
From your description, AE's Leave Color effect mioght do the trick. If the same color shows up elsewhere in the picture, you can animate a garbage mask to eliminate it.
Beneath the masked layer would be a non-masked layer using the same effect, but set to leave a color not seen in the picture, thus making the entire picture B&W.
There are extensive making-ofs on the Sin City DVDs and you can find tons of tutorials on Sin City/ pleasantville techniques. Anything from Leave Color to masking to fake color make-up.