Several ways to do this.
You can isolate a single color in the Secondary Color Corrector. Then repeat this for all the colors the subject is Wearing. This way you leave eg red, green and brown intact, while reducing the rest to greyscale. This assumes the subject is wearing colors significantly different to the field colors.

Another way would be to use a vector mask in the Pan-Crop pane to isolate the subject in one track, then in a duplicated track, remove all the color. Drawback is if the subject is moving round you have to do lots of keyframes.

You can do the above and automate it by using track motion tools from Boris Red or Hitfilm. HitFilm is cheapest and probably easiest to use.

I think John Rofrano has some excellent tutorials on this.
David