Isolated soap bubbles. The colours, produced just before the bubbles burst, are due to interference between light rays reflected from the front & back of the thin film of water making the bubble. The colours depend on the thickness of the film; black were it is very thin indeed, silver colours are produced by a film about 70 nanometers thick, gold about 100 nanometers thick, green about 150 nanometers thick & red as the film gets thicker.