Soaps come in different colours. But why is soap’s lather always white in colour?

Lather or foam is nothing but a large collection of small soap bubbles.

A soap bubble is, in turn, a very thin film of soap solution enclosing some air. Because of the low surface tension of soap solution, the film can stretch and spread and form innumerable bubbles with a very large total surface area.

Due to this, whatever slight tint is present in the thin film of the coloured soap solution gets subdued.

Although a soap film is more or less transparent, the lather or foam looks white because the light striking this large collection of bubbles gets scattered. That is the reason why all kinds of lather or foam appear white.

Source - The Hindu Speaks on Scientific Facts