To accomplish this effect in Final Cut Pro X, starting with a full color shot, you can use the Color Mask eyedropper tool to select the color you want to retain. Dragging the eyedropper expands the range of hue/saturation/brightness that gets included in the selection; and you can shift-click to add colors from other areas.
Of course, this process selects this particular color range everywhere in the shot. In our example, not just the red hat of the girl that we want is selected, but also the hat, scarf, and gloves of the girl sitting next to her. The View Masks button shows the selected areas in white on a black background, making the issue obvious.
To eliminate these other selected color range areas, you can add a Shape Mask that covers the hat of the girl that we want to keep red. So these two masks intersect, and we get just the selection we want.
However when I play the clip, both the girl and the camera move so that her hat moves outside the mask. To address this, we can animate the mask to move with the subject by setting an initial keyframe, moving the playhead, and repositioning the mask. The keyframe works on all properties of the mask so if needed it can also be rotated, it’s shape changed, and even its feather amount adjusted, and all those properties then animate.
Now that we have a mask that is tracked to the subject, we can perform the correction: in the Saturation pane of the Color Board, after selecting “Outside Mask”, reducing overall saturation all the way turns everything in the shot except for the area in the mask (the red hat) into greyscale values.
Now, if the oval-shaped mask that is part of the color corrector isn’t flexible enough, you can use the Draw Mask effect instead, which lets you create a mask of any shape. It requires first duplicating the clip as a connected clip but it’s well worth the extra step.
All the good details above. If you want to improve your color correction skills in Final Cut Pro X, check out our fully updated color correction tutorial.