It could still be a copper alloy, but maybe the composition isn't just right to call it brass, which is copper and zinc alloy (varying the proportion will alter the visual brass/gold tone). Now take that same copper and throw in some nickel and some zinc and you will have a copper alloy that is absolutely silver in color/appearance. As a matter of fact it used to be called nickel silver or German silver. They had to refrain from commercially calling it "silver" because there aint of bit of silver in it. I wouldn't rely on the scrape test but rather a PMI test to posi-toot-ly determine the shell composition.