system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are beer bottles usually dark green or dark brown?
gpt
Brewers started to use glass because it kept beer fresher, longer. However, there was one serious issue, if the beer was left in the sun too long, brewers found the beer would smell and taste skunky. Literally, it smelled just how a skunk smells. Scientists discovered that Ultra Violet rays from the sun break down the alpha acids in hops which then react with the sulfur present in beer to form a chemical that is almost identical to the chemical that skunks use to spray predators. Those damn UV rays. The solution was to tint the beer bottle to protect it from UV rays. Similar to how sunglasses help protect your eyesight; the dark brown beer bottle helps protect the beer from going bad. 

Why the green beer bottle then? Green beer bottles offer very little protection from the sunlight. Though, slightly more than a clear bottle, it does not help protect from UV as much as one might want. 

Around World War II, brown glass rose in demand and many companies had to forfeit their brown glass for their country. Unfortunately that meant companies with higher quality beers had to use clear glass, which made their beers look like cheaper, clear glass beers. Higher quality brewers’ solution was to sell their beer in green bottles so a consumer could tell the difference between a regular beer and a higher quality. The green beer bottle became a status symbol for many European breweries. These days, there’s not much of a reason to sell a beer in a green bottle other than for marketing and aesthetic.  Lucky for us, glass suppliers are able to apply clear, UV protected coats to glass that help keep beer fresh no matter what kind of bottle it is in.