Key out everything but one color? Ok I know this might be very basic for many of you, but I couldn't quite figure out how to do this ...

So, in case I want something like a reverse color keying where everything except one particular color is keyed out, what would be the easiest or, even better, most customizable way to do this in Premiere Pro?

I want to be able to manually set the tolerance level for similar colors as well as soft edges.

Thanks in advance!

No answers starting with "This is much easier with After Effects ..." please!
This is quite simple.  Use any keyer of your choice that can give you a good mask for the layer, then place an "Alpha Adjust" layer below it in the Effects control and click the "Invert Alpha" checkbox.  This will invert the alpha channel produced by the keyer and reverse the effect the way you are looking for.