The formation of colours in soap bubbles is due to the phenomenon of

The formation of colors in soap bubbles is primarily due to the phenomenon of thin-film interference. When light strikes a soap bubble, it reflects off both the outer and inner surfaces of the thin soap film [2]. These reflected light waves overlap and interfere with each other [1]. Depending on the film's thickness and the light's wavelength, the waves can interfere constructively (reinforcing certain colors) or destructively (canceling others) [2]. This process creates the vibrant, shifting iridescent patterns observed, which differ from the dispersion seen in prisms where light is split by refraction [1]. While white light contains all colors, the specific thickness of the film at any point determines which color is intensified by constructive interference [2].

Sources

1. [2] https://en.wikipedia.org/wiki/Thin-film_interference
2. [1] https://en.wikipedia.org/wiki/Soap_bubble