First you need to pick a clip that has one object’s color that is really different from the rest.
Then you need to find the “Leave Color” effect under the color correction folder in video effects.
Then you will change the “amount to discolor” to 100% to make the clip black and white.
Then you will click on the dropper next to the “color to leave” wording. You will then take the dropper to the clip and select what color you want to remain. (you can use the control/command button to widen the area you are selecting to 5 pixels)
You can also play around with the tolerance and the edge softness to get the desired look.