Although I don't exactly know HOW to do the color isolation, I can recommend the DVD for the movie Pleasantville. In the movie, a 50's B&W sitcom settings gets corrupted by 20th century teens, and things gradually start becoming color (cars, flowers, eventually people). There are some excellent scenes where B&W makeup is applied to a "colored" person's face that makes the face go back to being B&W. I recommend the DVD because in the special features there are some long talks with one of the cinematographers where he talks about how they set up some of the shots and the painstaking task of making sure the lighting worked both for color and B&W. He also talks about the makeup shot--they had to make green makeup that had the exact greyscale value as the flesh tone of the person's face, it's really cool special features, and a decent movie too.