Interestingly, a soap film can turn off the light if it has the right thickness (see an explanation of the phenomenon here). When shining white light on a soap bubble, some of the colours will be turned off and the colours you see in the bubble are the remaining complementary colours. The best way to see that the light really is turned off in a soap film is to shine on the bubble with light of only one colour (so-called monochromatic light). Here a few examples are presented. The soap films are black were the light is turned off.
NB: Click on the images to see them in full size
In the following it is illustrated what happens when a few colours are combined.
See also our photos of soap bubbles illuminated with white light which contains all colours.