its not that simple as it sounds
dont know how they do but if i have to do that
i will save all frames as png open it in photoshop and make
selection for each frame then apply some commands to make
background black & person will stay colored
Ok this is how to do it. You will need a bunch of masks, or one mask, depending on how you have shot and what you have shot. Depending on the software you are comfortable with, the actual tools will be different. Tell us which software you wanna use, this is how to do it in Premiere.
Add the "Leave Colour" mask in Video Effects. Select the colour you want to preserve, all other colours go b/w. You will need to increase the contrast before, you do this, or the image will look too dull. To highlight just the indivudal, you will need a bunch of masks, this is for the hair, the skin, and the clothes AT LEAST. If the shirt has a colourful logo, you are in trouble. Then you will need to add a 16 point garbage matte around the logo on the shirt, and keep two layers of video, one for the b/w version, and one for the colour version. Also, preserving reds and yellows are a little crazy because of the skin colour. A person with a red/yellow/orange/brown shirt against a blue-green background is the easiest and smoothest to do. If you can post one image from the video you are going to edit, I can give you the settings here.
If you are filming, the professional cameras let you do this when you are filming itself, and this is how it is mostly done in ads and movies, as Rhitwick pointed out.