It's anodized aluminum. But they don't publish what the specific alloy used is. This Apple Patent article gives some details on the other elements contained in the various layers.
In 2018 Apple revealed that the MacBook Air's enclosure was using 100% recycled aluminum. This revelation was met with thunderous applause from the audience. Our report presents the video of that keynote segment covering this single point.
www.patentlyapple.com
You could likely get close with aircraft aluminum and having it anodized to the color you want.