"...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...."
If the layer is uniform the colors do not change at different angles - you can try that on a binocular lens - usually those coatings, if bluish, are 1/4-wave
An oil drop on a puddle will demonstrate different colors because it isn't uniform thickness. The oil has the same refractive index over the entire spread-out drop but its thickness varies and the interference fringes consequently have different colors.
Certainly if there's a big "glob" of oxide on the surface that will act as a real pigment as far as color goes.
