Using Keying to select a color and darken it

So, I apologize in advance, but in viewing tutorial after tutorial, it's all about green screening.

That's not what I'm trying to do. I want to use some utility to select a given color in a video and just darken it.

I've tried masking, but the scene is too complex. But if I could create a matte selection by selecting the color, and then apply a mask to that selected color and darken that color, that would be fantastic. I know you can do this in photoshop.

Can this be done in fcpx? It seems like it should, but I can't figure it out.

Any help would be appreciated. Thanks.

Edited: Never mind, figured it out. Sorry....!

Glad you have figured it out. The FCP X Color Mask can indeed be a very handy tool.

- Ronny