After watching Avengers again

I think anodization is more likely than PVD or CVD (Chemical Vapor Deposition). Although, I'll admit, I'm not familiar with how the colors are achieved during anodization. Cool! I get to read up and learn a new thing!

I think that for the purposes of the prop, it's an aluminum shield that's anodized... but in the story canon, it's actually some bizarre comic book science thing.

Indeed