Search the FCPX Help system for- Target a specific color using a color mask.
A color mask isolates a particular color in the image. You can apply a color mask to a clip to correct a specific color, or to exclude that color from corrections to the rest of the image, or both. For example, you could mute a brightly colored shirt in the background that distracts attention from a clip’s main subject.
Using a color mask to control the color correction allows you to pick a color and then independently adjust that color (reduce, enhance, or change it) or adjust everything except that color (for example, add a tint or reduce the brightness or chroma levels).