Pour a stout into a clean glass and you can hold the two colors side by side: the liquid is nearly black at the center and ruby where the light slips through, while the head sitting on top of it is white. Nothing was added. The foam is made of exactly the same liquid that looks almost opaque below it, plus air. The difference between amber beer and white froth is not chemistry but geometry. In the glass, light travels through centimeters of beer before reaching your eye, and over that distance the liquid's pigments strip out enough of the spectrum to read as amber. In a bubble wall, light crosses a film only a fraction of a micrometer thick, a filter too thin to remove anything you would notice. What the walls lack in tint they make up for in numbers: they bounce light around until it arrives at your eye from everywhere at once. It is the mirror image of the trick that turns clear absinthe milky when water hits it - in the drink, droplets are created where there were none; in the foam, the interfaces are the whole structure.
A wall of liquid thinner than a wavelength
Beer foam, soap suds, and whipped egg white are all the same kind of object: gas pockets packed together and separated by walls of liquid so thin that several of them stacked up would still be thinner than a single wave of visible light. At every one of those walls, light meets a boundary between liquid, gas, and liquid again, and at every boundary some of the light is reflected rather than passed through. One reflection is invisible. Multiply it by the millions of films in a pint of foam and the effect becomes overwhelming: light enters the foam, ricochets from wall to wall, and loses all memory of where it came from long before it leaves. Physicists who study foams with laser methods treat the material the way they treat fog or milk - as a diffusing medium, where light travels by Brownian-looking steps from one scattering event to the next rather than in straight lines.
This is why a foam's whiteness survives every change of liquid. The color of the liquid sets how strongly each crossing of a wall tints the light; the number of walls sets how often the light's direction is reshuffled. In almost every foam you will ever pour, the reshuffling wins by a wide margin.
A short path is a weak filter
Color by absorption is a game of distance. The amber of beer, the gold of apple juice, the brown of brewed coffee all come from molecules that absorb particular wavelengths, and each millimeter of liquid takes another small bite out of the passing light. That is why a deeper glass of the same beer always looks darker than a shallow one. A bubble wall in a fresh beer head is on the order of a hundred nanometers thick - roughly one ten-thousandth of a millimeter - and across such a short crossing even a strongly colored liquid removes only a vanishing fraction of any wavelength. Whatever faint selectivity each crossing contributes, it applies almost equally across the spectrum for the tiny distance involved, so the light that has been scattered back out of the foam keeps a nearly balanced mixture of wavelengths.
A balanced mixture arriving from all directions is precisely what your eye reports as white. It is the same reason a fair-weather cloud is white even though water itself is nearly colorless, and the same reason a wave of sea foam can paint a beach white regardless of the water's tint. The color you see is a property of the light-traffic pattern, not of the tinting molecules.
The same trick in the sink, the shower, and the meringue
Once you know the mechanism, you find it running everywhere in the kitchen. Dish soap dyed an emphatic green still produces white suds; shampoo in any color you like whips up into a white lather; egg white is a transparent, almost invisible liquid until a whisk folds air into it and it turns opaque white. Whipped cream is the same story with a small twist: cream starts out ivory rather than clear, and its fat droplets scatter light even before the bubbles arrive, which is why the finished topping reads a soft warm white instead of a harsh one. In each case the coloring molecules are still present, and still doing their job - they simply never get the running distance they need.
Wet foam behaves slightly differently from dry foam, and the exceptions confirm the rule. A young, wet beer head is mostly liquid with round bubbles, so light crosses a bit more material between scatterings, and on a very dark stout the foam can carry the faintest ivory or oatmeal cast. Crema on espresso goes further: it is a foam of coffee oils and dissolved compounds, and it reads tan or hazelnut because the liquid trapping the bubbles is genuinely brown rather than amber-clear. Give the foam a moment to drain and the walls thin, the scattering strengthens, and the color drifts back toward white.
Reading a glass of beer correctly
None of this changes the flavor, but it does change what your eyes can honestly tell you. The head is not a different substance floating on the beer; it is the beer, reorganized into a geometry that trades color for light-traffic control. When a brewer talks about foam stability, they are talking about whether those thin walls can hold their shape against drainage and collapse long enough to keep the white collar on the glass - a question of surface tension and bubble size, not of tint. So the next time a nearly black stout produces a pale cap, treat it as a small demonstration of optical bookkeeping: the liquid keeps its color, the foam keeps none of it, and the difference between them is the length of the road the light had to travel.