Rainbow-colored oil films on water only form, and are visible, under specific circumstances. The oil has to be "stretched" to be a very thin layer, not forming globules, thin enough to cause diffraction at visible light wavelengths. Furthermore, parts of the film must be thinner than others: it is not at equilibrium. I achieved this rinsing (no soap) a oily pot, continuously filling the pot with hot water and allowing the water to overflow over the edge, taking some of the oil with it and stretching the rest.
The light has to be positioned to see reflectance.
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