Why do all the soaps with different colours produce the same colour of foam?

Soap foam is a collection of bubbles, which are simply air trapped in a very thin film of soap.

You can take any colourful soap and make bubble or foam out of it. Due to the very thin film, we cannot see the colour in the bubble. We can imagine this phenomenon with a balloon. When we blow a balloon to its maximum size it looks lighter in colour. Due to the stretching of rubber in balloons, the colour is seen lighter.