After searching for a way to simulate the color changes found on lab grown bismuth crystals and dichroic glass, I eventually happened across Pruster's thin-film interference OSL shader. With the help of various literature and the OSL shader, I was able to adapt Pruster's script into a GPU compatible interference node group. Find Pruster's original news post here.
The interference group takes the complex refractive index of a substrate and up to two thin-film layers to calculate the physically-correct colors of the desired material. With it, materials such as aged copper, 'a thin-film on an arbitrary dielectric', car paint, and heated iron can be simulated correctly as seen below:
The interference group also has the option to simulate plain conductors such as gold and osmium:
And while we're at it, let's look at the golden reflections of a space helmet material and the shiny pastels of a dichroic glass material:
Only the interference node group is under the CC BY-NC-SA 4.0 license, but its output can be used for any purpose. All the materials shown here are only a small preview of what is provided in the blend file, so check it out and maybe even create your own materials with the interference node group.
Hope you enjoy it!