The pat answer is that white light is mixture of all the colours and the size and number of the bubbles gives a large number of fine reflecting surfaces which gives an averaging effect making things look white. However, thinking about it some more, the question leads on to deeper questions. If you took a spectral analysis of light from a foam, how complete would the spectrum be? Is the analysis for heads of Guiness consistant across all pints and is it the same or different to washing-up liquid foam? Dunno ...