Interesting, it makes me think of an old Schaeffer I have that has some sort of treatment on the metal of the body that gives it a rainbow hue, sort of like a thin film of oil on water. This makes me wonder how do duplicate that effect.
It doesn't take much to etch aluminum. Do watch the vapors though.
The treatment on the Schaeffer sounds like it works the same as anodizing on titanium. A clear coating of varying thickness is applied. The color comes from lightwave interference allowing only certain colors to emerge. This is exactly how a soap bubble gets its color. It can be done by starting with a mirror like surface and adding a film of metal that gets thicker to one side. This is done in a vacuum and is done to some sunglasses and lawn ornaments and other stuff.