Masking with Clown Passes

Hi,
I'm new to Fusion and am just starting to get my head around things.
My work isn't related to the film industry in any way, but would have good use of it in product development.
I have to work with Keyshot, which unfortunately doesn't give all the render passes one would usually expect. So for masking my best bet is a clown pass:
I'd be really thankful, if someone could explain to me how to make my masks out of this type of pass. I tried the Wand, Chroma Keyer and some others, but maybe my mind is still stuck within other workflows.
Thanks,
David
I'm new to Fusion and am just starting to get my head around things.
My work isn't related to the film industry in any way, but would have good use of it in product development.
I have to work with Keyshot, which unfortunately doesn't give all the render passes one would usually expect. So for masking my best bet is a clown pass:

I'd be really thankful, if someone could explain to me how to make my masks out of this type of pass. I tried the Wand, Chroma Keyer and some others, but maybe my mind is still stuck within other workflows.
Thanks,
David