There isnt an easy way to do this that I know of, and although I havent really used it much this seems like something that discreete's combustion is probably capable of. Doing it frame by frame is always a possibility..
One way that I have used if the subject you want to leave in color is either brighter or darker then the rest of the scene. Adjust the brightness and contrast of the video so the subject more or less becomes a black. If you have to, invert the color... Then use that now black and white copy of the video as a mask for a 2nd copy of the video. Now you should see JUST the subject during playback and the rest of the scene should be gone. Finally take a 3rd copy of the video stick it in the background and de-saturate it. It works but definitely not all the time.