A common fallacy is that dark beer is stronger than lighter beer. This is often true, but color is not a result of high alcohol content. It's determined by the type of malt a brewer chooses to use.
Case in point: Guinness Stout, one of the darkest beers out there, is only 4% alcohol by volume, putting it roughly on par with watery old Coors Light(4.2%) for strength. In contrast, Pyramid IPA, of medium hue, packs a robust 6.7% abv wallop.
In order to attain the classic dry taste and opaque creaminess of Guinness, the brewery uses a dark roasted malt (I forget which one. Bruce could tell me). The grain is already a deep brown, almost black, when it goes into the kettle.
Alcohol content is regulated through the yeast. As yeast eats the sugars in the malt (fermentation), it converts them to alcohol. A brewer can stop the fermentation when the appropriate alcohol content is reached.
TO balance malt sweetness, hops are added, which give many beers that tantalizingly bitter aftertaste and floral aroma.
The flavor and color of beer is a complex juggling act, and paying attention to these things, instead of just the alcohol, infinitely heightens the enjoyment. Maybe I'll have a Guinness with dinner tonight.