Whenever you see petroleum oil spilled on a wet surface, it turns rainbow colored. That doesn’t happen with olive oil, or other liquids. Why do petroleum products change color that way?
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Others have already explained the mechanism of _thin film interference_, so read those posts for the physical mechanism. Instead, I want to talk about what you called “rainbow colored”:
The standard [rainbow](https://upload.wikimedia.org/wikipedia/commons/thumb/2/27/WhereRainbowRises.jpg/640px-WhereRainbowRises.jpg) is sun (or any other fully white) light split into its colors, sorted by wavelength: red, orange, yellow, green, cyan, blue, purple. Invisible for us, it is bordered by infrared and ultraviolet at the two ends.
But what [oil-on-water](https://qph.cf2.quoracdn.net/main-qimg-0b09e1839766fa1a471366063e4b3738-lq), [soap bubbles](https://media.sciencephoto.com/c0/27/96/76/c0279676-800px-wm.jpg) and [bismuth crystals](https://i.ebayimg.com/images/g/ve0AAOSwuVpcutz5/s-l1600.jpg) produce is decidedly **not** a rainbow in that sense:
[Here is a picture of the theoretically resulting colors](https://i.stack.imgur.com/AzsSJ.png), and here some more [photographs](https://physicsopenlab.org/wp-content/uploads/2021/09/thinFilmscover.jpg) [of](https://sciencedemonstrations.fas.harvard.edu/files/science-demonstrations/files/thinfilminterference-freeze-bottom-3.jpg?m=1446824435) [the](https://i.ytimg.com/vi/WTxDyYHaYAI/maxresdefault.jpg) [real](https://www.soapbubble.dk/content/2-artikler/9-filmtykkelse/farver-i-saebehinde-cover-lille.png) [thing](https://sciencedemonstrations.fas.harvard.edu/files/science-demonstrations/files/thinfilminterference-freeze-top-2.jpg?m=1446824437).
You get: black, gray, white, orange-brown, purple, blue, light-blue, yellow, pink, blue, green, yellow, pink, green, pink, green, pink, green, pink, …; with it slowly fading into gray further on.
This entire process is similar to how harmonics works for good music, we get those patterns that are not as simple as a rainbow (or playing the chromatic scale). Instead, we get those even more intricate pseudo-rainbow.
Note: There actually is some variation depending on the materials (or more accurately, on the speeds of light in them), not all give exactly the same. But they are close enough, especially early on.
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