Ultraviolet (UV) light is the enemy of beer. The more UV a bottle will block, the less likely your beer is to go skunky, otherwise known as light struck flavour (LSF). When the sun hits your beer, hop-derived molecules, called isohumulones (essential in the stabilising of beer foam) decompose into a reaction catalyzed by riboflavin. If you're meticulous about beer, you're aware of International bittering units (IBUs) which denote how bitter a beer is. This IBU number corresponds to 1 part per million of isohumulones! The catalyzed reaction creates 3-methylbut-2-ene-1-thiol ('skunky' thiol). We don't want this to happen, so we keep light, and much UV as possible away from our beer, as it can go bad in just seconds!
Some craft breweries are moving to completely opaque bottles, for example Rogue brewery offers their higher IBU beers in ceramic 750ml bottles. Restaurants that care about the quality of their beer will offer your beer shade when being served outdoors.
So next time you're picking up some beers, make sure they are in a vessel that protects from UV; not the green or clear bottles that are just asking for trouble. If you're enjoying a beer in the outdoors, make sure it's covered from the sun!
Homework: What's the IBU of your favourite beer?