Because the light waves are out of step, some of them are cancelled out. If one wave is up (crest) and the other wave is down (trough), then they cancel, and you don't see any of that color light.
The thinnest part of the bubble will look black, with all the light canceling. You see that just before the bubble pops. If the film is a little thicker, it will look yellow. A little thicker it will look magenta. A little more lets you see blue, and even more looks blue-green.
If the film is even thicker, then you go back to yellow again, and then magenta, blue, and back to blue-green, and so on.