You can use the Hue/Saturation effect to desaturate a single hue range (it's actually more accurate than that, but might look blocky).
Apply Hue/Saturation to your clip. Add an Effects Mask > Color Mask. Access the mask from the small rectangle with the circle inside that appears when you mouse over the effect label bar:
Use the eyedropper tool in the Viewer just like you would in Photoshop and click on the problem area. Hold down the shift key if you need to expand the selection (just like you would in PS). Adjust the Saturation.
There are two types of Color Mask: 3D and HSL. I recommend you start with 3D.
You can "smooth" out the selection with the Softness control if using the 3D type color mask.
If you decide to us the HSL type (it is more accurate and *can* produce much more refined results), you adjust the ranges with the top slider controls on the HSL bars and "soften" the ranges with the slider controls under the HSL bars:
They're a little difficult to grab sometimes. As you might be able to tell, you can turn on and off the H, S, and L sliders to set up the control in various ways (like all the same hue regardless of saturation or lightness, or in combinations of your choosing).
You can add **as many** Hue/Saturation effects to a clip as you need and each one can have its own separate color mask. You can also apply and use **as many** Shape Masks as garbage mattes (either limit or add regions to extend the use of the effect) if you need. (You only get one color mask per effect added).
So yes — there is a "photoshop way" of doing this in FCPX. Unfortunately, Hue/Saturation is just one of the very few effects provided with FCP that are useful in this way. There are not a lot of options with the default set of effects. [I've built an entire library of tools designed to work in this way in FCPX and nobody is interested because the tools you get in NLEs is "just how it's done" in video.]
