Not interference but diffraction. Fine parallel grooves send each wavelength off at its own angle, splitting white light into a fan. Because the wrapper is creased, every facet aims its fan somewhere different — which is why it flashes rather than glows. A flow wrap makes that harder than a twist does: the film is pulled tight over a slab, and a flat face returns one angle across the whole of it. What carries the colour here is the gathering — the creases that run diagonally off each crimped end, where the film was dragged into the sealing jaws.