The soap bubbles are very thin, only a few molecules thick. As light hits this, part of it is reflected from the surface, and part of it goes into the bubble. At that point, part of it goes on inside the bubble, and part is reflected back toward the outside. The interference between these waves causes you to see light and dark bands. As the light passes through the boundary, it is bent at a slight angle. Different wavelengths of light (different colors) bend at slightly different angles. As you look at the bubble, the different colors are broken up because you are seeing different areas of the bubble at slightly different angles. This is the same mechanism that makes a CD appear as all different colors. The same as a diffraction grating also, but the bubble is curved and usually liquid and chaning over time.
John - The Eternal Pessimist.