It looks like you may be looking for masking, that is not currently available in LumaFusion.
Take a look at the example below:
It uses 2 copies of the same layer of video. The bottom one is set to black and white. The top video has been cropped to a subject and had the edges softened.
In your case you need it the other way around. The subject is black and white and the background is still in color.
I hope this helps.
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