Check out what I did with BobsColorReplace.
Its a short clip that shows some of the hidden
power in these filters.
You can see the file at
(I finally took a while for me to play with this.)
Took me about 10 minutes to set up and I think the results are
pretty good. Just use white as the destination color
and invert the mask for those that already have it.
Oh and restrict the area you are working with.

Its Just Bob