Beer! The head on a glass of beer is a different color than the body, and yet the chemical make-up is the same. Why?
Pour a glass of beer, and the body will be a dark, rich color. It's a color that's warm, and inviting. It promises to make you forget about all the troubles you have . . . for a little while. Just a little while. The color is the result of the way light interacts with the composition of the beer. The molecules of beer absorb some wavelengths of light, and let other wavelengths through to your eye.
The head, on the other hand, is foamy and pale. The body and the head of beer are made of the same stuff, so why is the color so different? I've heard people claim that it's the ratio of air to beer that we see that makes the difference. After all, if the beer is churned up enough that it is full of bubbles as well, it turns light-colored. This doesn't explain, though, why pale ale and Guinness have similar-colored heads and massively different colored bodies.
It's not the composition that changes the color of the head, it's the structure. The head of a glass of beer is made up of tiny bubbles. This makes incoming light hit many curved surfaces. As anyone who has ever spent a lazy day idly looking at how the curve of a glass, or a fishbowl, or a clear marble scatter light knows, this means that the light is diverted off course. One curved pane means only a slight diversion for a beam of light. The bubbles in a head of beer present it with hundreds.
This is a phenomenon known as Mie Scattering, and it happens in sea foam, soda foam, beer foam, rain clouds, or any other series of curved surfaces which baffle, frustrate, and thwart the straightforward movement of light. The light zooms around inside the foam, getting diverted again and again. Sometimes the path it takes is so radically curved that the light comes back up to the surface again, and makes straight for the eyes of the nearest drunk.
But why does the scattering change the color of the foam? It's still going through beer, not water or soda. The foam is white or whitish, regardless of composition, because all the wavelengths of light are scattered equally. The light that comes from one patch of foam, or another, is made up of all the different wavelengths of light, thrown together randomly. That combination of wavelengths results in what we consider white light.