That’s that rainbow-like effect you may see on soap bubbles, oil spots or metals. Koru simulates it pretty well, but you need to specify some wavelengths… No worries, it is not that difficult. Let’s start with a metallic material that we made here:
Now check the thin-film interference box below the specular block and see the difference:
Koru added a thin (200 micro-meters) film with the index of refraction of 1.5 and simulated light interference there. Adjust the width to see different colors, here we set it to 250 micro-meters:
Best of all, you can load a texture controlling the thickness in different areas. Here we loaded a built-in noise texture:
Play with the thickness levels, controlling texture and N/K values in this block to get the effect you need. It may take time to master that, but it is worth it.