Joe, I might be doing something wrong on the layers...here is what I did:

Original sepia photo is background layer...
I duplicate that and called the new layer color
Used magic wand to select the necklace strap and set feathers to .5
I filled the selection with red.
Set this COLOR layer to Difference which seems to make the picture all black instead of the necklace strap.
I duplicated the color layer again, put it on top, named it color2 and set it to multiply but I still ahve a black picture with the red strap.

what am I missing?