Iridescence
Color that changes depending on the angle you view it from, like an oil slick, a soap bubble, or a beetle's shell.
See it
What it is
Iridescence is color that comes from structure instead of pigment, and there is more than one structure that does it. Thin-film interference is the soap bubble and the petrol slick: light reflects off both the top and the bottom of a film only a few hundred nanometers thick, and because the two reflections travel different distances, some wavelengths cancel and others reinforce. The film does not move when you do; tilting changes the angle light takes through it, so the path length changes and a different set of wavelengths survives. Beetle shells, peacock feathers, and abalone stack many such layers into multilayer structural color, and a CD is different again: its evenly spaced pits are a diffraction grating, fanning the spectrum out by angle rather than by film thickness.
In 3D you get it honestly with a thin-film or multilayer shader that maps view angle and layer thickness to a spectrum. A Fresnel term alone will not do it: Fresnel only says reflections get stronger at glancing angles, so it changes how much you see, not what hue you see. It is a multiplier over a spectral model, not a substitute for one. On a flat page there is no real viewing angle, so you fake the input: drive a hue shift from cursor position, scroll offset, or device tilt, and layer a soft multi-stop gradient over the base surface with mix-blend-mode: overlay or soft-light so the underlying material still shows through.
The mistake everyone makes is reaching for a full-saturation rainbow. That reads as a pride flag or a gaming keyboard, not as a material. Real iridescence is a narrow, biased band (cyan into magenta into gold), it stays desaturated near the base color across most of the surface, and it concentrates at glancing angles, which on a 2D shape means the edges.
Ask AI for it
Give this surface an iridescent, oil-slick finish: layer a soft multi-stop gradient sweeping cyan into magenta into gold over the base color, blended with mix-blend-mode: overlay so the base material still reads through. Keep saturation low in the middle of the shape and let the color concentrate toward the edges, like a glancing-angle reflection. Shift the gradient angle and hue with cursor position so the color moves as the user moves, and avoid an even full-spectrum rainbow.