Sophia du Plessis, of Dunedin, asks :-
Why is it that no matter what colour bubble bath is used the bubbles are always white.
Jevon Longdell, a physicist at the University of Otago, responded.
To answer this question we need to look at how a soap bubble interacts with light. As far as the optical properties are concerned we can treat each of the bath bubbles as being made of a thin film of the bath water.
Bath water interacts with light in two different ways, firstly as light travels through the bath water it slowly gets absorbed. Adding a dye increases the absorbtion for particular colours and this is what makes dyed water look coloured. The other effect is reflection. When light crosses a boundary between air and water a few percent of light gets reflected. This is why you can see reflections in a puddle on a still day, or from window glass especially when looking from a bright room into the dark outside.
If you are looking at a big thick layer of dyed bath water, like a bath with no bubbles, then it is the absorbtion which has the biggest effect on the colour.
However soap bubbles are thin, a few thousandths of a millimeter. This means that the reflection from the soap bubbles is much more important than the absorbtion. The bubbles just aren't thick enough to absorb much light. Because the light reflected from the foam contains all the colours, it appears white.
In order to make brightly coloured bubbles with dye you would need lots of dye, enough so that a good fraction of the light got absorbed in the very small thickness a soap bubble. However you would have to use so much dye, you would come out of the bath brightly coloured also!
Individual bubbles can look coloured due to interference, caused by to the wave nature of light and the fact that bubbles are so thin. However interference doesn't make the foam coloured, the rainbow of colours from all the bubbles add together to make white.