Shocked that nobody here knows the answer. Lots of wrong answers. Aluminium is my favorite one.
Lamp cord and bulk cheap speaker cable have been marked this way for decades. It is sold as copper wire. The silver colored one is usually made into the negative since copper seems to be reddish. The silver colored one is Sn coated.
Nobody has ever bragged that this is superior in my experience. it is generally less espensive to just just one red wire and one white wire then it is to mark the insulation. I know when I was young I learned quicly to make sure the wires were coded in some way so you didn't have to get out a meter to figure out which wire was which. Some really cheap wire wasn't marked.
Jerry