If I'm not mistaken, thin-film anodization one more example of this: https://en.wikipedia.org/wiki/Anodizing https://en.wikipedia.org/wiki/Anodizing - "Anodic

If I'm not mistaken, thin-film anodization one more example of this: https://en.wikipedia.org/wiki/Anodizing https://en.wikipedia.org/wiki/Anodizing - "Anodic films can also be used for several cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add reflected light wave interference effects. [...] The colour [sic] formed is dependent on the thickness of the oxide (which is determined by the anodizing voltage); it is caused by the interference of light reflecting off the oxide surface with light travelling through it and reflecting off the underlying metal surface." There's one more example in metallurgy I don't remember the name of: where you can get metals to spontaneously form color gradients by heating them in a specific way. Like, there's a thermal gradient, and that somehow translates to an oxidation-thickness gradient, which makes a rainbow. (?)