Thin-film interference and iridescent colors

What you are seeing: reflectance of a thin oil-like film (n = 1.33) on a glass substrate (n = 1.5) viewed at normal incidence by white light. The film causes wavelength-selective constructive and destructive interference between the front and back reflections. The bottom strip shows the reflected spectrum (sum of reflectance times blackbody-ish illuminant) translated to RGB. The top curve shows $R(\lambda)$ for the current thickness.

Move the thickness slider and watch the reflected color cycle through blue, green, yellow, red as the constructive-interference wavelength $\lambda = 2 n_{\text{film}} d / m$ sweeps across the visible spectrum. This is why oil slicks, soap bubbles, and butterfly wings show iridescent color: even a slight thickness gradient produces a rainbow of reflected hues.