....The fact that different compositions can cause the same color is because the colors are optical interference colors and not due to the intrinsic color of the surface layer.
If you look at (not through) your camera or binocular lens you will see a blue or reddish color - that is NOT a pigment, but an optical interference color produced in the same way colors are on case-hardened guns....
I think that's what Rocketman was saying. Supposedly, it's not the color of the 'stuff', it's the thickness of the layer that make the colors appear to be different.
Maybe though, there is a 'pigment' effect. The Gaddy article specifically points out that case colors do not change appreciably when viewed at different angles, which happens with your optical interference example.
Heat affected steel, like temper colors, seems to show changeable colors due to minute differences in the thickness of the iron oxide, but is otherwise a 'flat' layer. Case colors seem to have a component of optical interference, but those opaque 'globs' of iron oxide described in the article seem to be of big enough size to be a pigment and have some contribution to the final color.
In the end though, it seems to be just one ingredient, iron oxide, with very little other trace components. Interesting stuff. Not trying to be contrary, seems that true case colors are more complicated than eek torch colors.
