Just curious.
How can you make only select parts of a video in color (ie: one person)?
Do you have to overlay two videos?

I guess the first thing to ask is what kind of editing software are you using? If it's Final Cut then you use the 3-way color corrector.

you could always try green screening that one person, and replace the screen with the other footage in black and white. that's the simplest way i could think of, though it probably wouldn't be the highest quality of effect.

I made a film using it in Final Cut Pro. Its actually really nice looking. If you select three way color corrector, and then open the limits effect sub menu, you can use the eye drop tool to isolate a color... Hold down shift and press it on another color to get two in there. Hope that helps

it's easy, you need to isolate that color with a color corrector and then desaturate everything else.