The demonstration just performed is a classic way of showing how sunsets work.
White light is comprised of all of the colours of the rainbow i.e. the light spectrum.
The different colours you see represent the different wavelengths of light.
A orange object is only 'orange' when you see the orange wavelength of light being reflected off it.
When light travels through a substance, some of the visible light wavelengths are absorbed whilst the other light is reflected. As the light entered the milk solution, the light in the blue end of visible spectrum was scattered by the suspended milk solids. This left the lower energy wavelengths of orange and red to pass through the solution, creating the orange/red colour seen in the experiment.
So how does this relate to sunsets?
The daytime sky appears blue because this blue light is scattered more readily towards us, known as Rayleigh scattering. During sunset the sunlight is still scattered, however the blue light is scattered away from our eyes leaving the oranges and reds you see.
So how does this relate to blue icebergs?
In an average iceberg there are large amounts of trapped air bubbles. As air scatters light, passing light through an iceberg scatters all the visible wavelengths of light toward you.
An iceberg formed under water away from the air will have little to no air bubbles within it.
Passing light through such an iceberg will have the weaker red wavelength of light be absorbed by the ice, leaving the more high energy blue light to pass through the iceberg... so you see it as blue.
Mixing coloured light together always makes white; known as colour addition.
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Reference: Kane, J. W. Sternheim, M. M. (1988). Physics. 3rd ed. John Wiley & Sons, New York.
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