White light, such as from the sun or from a light bulb, contains light of all colors. Light has waves, and the length of the wave, from crest to crest, determines the color of the light. When light reflects from a bubble, some of each wave reflects at the outside surface of the soap film. Some light travels through the soap film, and reflects from the inside surface of the film. Interference between waves occurs whenever waves travel through the same space.
Interference occurs when two rocks are tossed near each other into a lake.
Circular waves on the surface of the water spread out from where each rock entered the water. Where the crests of two waves meet, interference between the waves causes the motion of the surface of the water to increase. Where a crest and a valley meet, interference reduces the motion of the water's surface. Similar interference can occur in waves of light. Waves of light reflected from the inner and outer surfaces of the film of a soap bubble can interfere with each other.