Physics of Bubbles

Soap bubbles have an iridescent quality that changes color over the lifetime of the bubble. This happens because light is reflected from the inside and outside surfaces of the bubble. As the bubble's surface becomes thinner due to drainage or evaporation, the light reflection changes. The colors change because when light hits the surface of the bubble, some of it is reflected off of the outer surface of the bubble. Some of the light passes through the outer layer and is reflected by the water layer or the inner layer of soap.

The color that is visible depends on the interference between the two rays of light. When the waves exhibit constructive interference, the color is brighter. When the waves exhibit destructive interference, the color becomes duller. As the bubble becomes thinner over time, the waves tend to have more and more destructive interference, which is why bubbles look almost black right before they pop.

All images from: http://www.webexhibits.org/causesofcolor/15E.html