Soap film walls are typically only 100-1000 nanometers thick, often thinner than a wavelength of visible light. Light reflects off both the outer and inner surfaces, and those two reflections interfere constructively or destructively depending on thickness and viewing angle, producing the shifting rainbow bands. Gravity drains the film thinner from the top down, and once a patch nears zero thickness it goes optically black just before the bubble pops. Stable stretchy films are only possible because surfactant molecules lower the solution's surface tension to roughly 25-35 mN/m, well below plain water's ~72 mN/m.