Why does a bubble go multi-coloured?

A bubble appears multi-coloured due to the phenomenon of thin-film interference. The soap film of the bubble varies in thickness, causing different wavelengths of light to reflect at different angles. When light waves reflect off the inner and outer surfaces of the film, they can interfere with each other—constructively or destructively—resulting in a spectrum of colors. This creates the vibrant, shifting appearance of colors on the surface of the bubble.