You cannot, generally, solder Aluminum to Copper. Aluminum creates an oxide barrier as soon as it is exposed to air.
Where do you get the wire form? Some dark silver wires may be silver plated copper, which can be soldered to copper easily. Dark silver coated wires are dark because the silver tarnishes, just like grandma's silverware, but it is still conductive and can be soldered with good resin flux solder.
You can tell if a wire is plated copper if you scrape off the finish enough, you will see copper beneath. If you still see silver color only, then odds are it is aluminum wire, which is of little use in hobby electronics (in general)