Red brass is brass that contains 85 percent copper and that is why red brass is more valuable for scrap metal than yellow brass.
Yellow brass is brass that doesn't have as much copper content in it and has more less valuable metals alloyed with the brass when it's made.
Yellow brass also needs to be purified more when recycling it to remove those less valuable materials that are added to the yellow brass.
Yellow brass is still worth quite a bit for scrap metal but the red brass is worth more due to it's copper content.