The brown colour comes from malt, which is produced by allowing barley grains to germinate and then roasting them. A low roasting temperature makes a light beer. A higher temperature makes a darker beer. Just be thankful that you aren’t living in the 19th century, when some publicans tried to shortcut the process by adding concentrated sulfuric acid to their light-coloured beers, carbonising the sugars to produce an instant dark brown colour, and instant stomach problems in those who drank it. The bubbles in the head are surrounded by a film of liquid beer, but the film is so thin that it can’t absorb enough light to affect the colour of white light as it passes through. White light reflected from the surface of the bubbles also stays white, giving it its overall white effect.