Why do coloured soaps produce white bubbles?

Foam or lather 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 in 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. Because of this, whatever slight tint 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 why all kinds of foam look white.