I have also seen the copper solder in the Metalliferous catalog.
I have found that the gray color blends better with the
copper–not as eye catching as silver solder. I also have
noticed that it gets a copper flashing on it after soldering,
blending perfectly. I usually have to clean up the seam, and
lose that copper coating,amd have found the gray inoffensive
enough (it patinas well) but it seems like one could experiment
with achieving it purposefully–reheating the finished piece.