I love seeing iridescent colours on oily water or soap bubbles. It comes from the surface of the liquid being only a few nanometers thick. The top of a bubble will be thinner than the bottom of it as the liquid moves down with gravity. There are two surfaces, inside and out. When light hits the surface of a bubble it can reflect off the top and bottom of the surface and you get interference patterns… Some reflections enhance the colours and others can cancel each other out… This is what causes iridescent or rainbow colours.