Key a color other than blue or green?

MacOS 10.14.4

Final Cut Pro X 10.4.4

Is there a way to key a color other than blue or green? Similar to picking an alpha channel color in image editing programs?

The color wheel on the FCPX seems to lock in around blue and green, and I need to remove what is basically a dark grey/near black background (and the blue/green key is removing things I don't want removed).

Thanks for any help/advice