Red Barrels are a video game cliche'. Everyone knows if you shoot, they blow up. The people making the video game Bulletstorm wanted exploding barrels in their game, but they dreamed to make their barrels green. Then they woke up.

People Can Fly put green barrels in their game and after they tested their game on real human beings here is the general consensus;

In the beginning we had green barrels, but people didn't get it right away.

They got completely ignored by the players and no one guessed or assumed that they were explosive. Why not? Because they weren't red. Everyone knows that only the red barrels are explosive. There's a lot of stuff going on in Bulletstorm at the same time, so it's vital for us that the player can quickly read the environment and act intuitively. This is especially true since you can leash and kick objects as well so they can be flying all over the place. There's no time to analyze objects on a detail level, so the shape and color have to be enough. It became apparent for us that the most efficient way to communicate its purpose was to make it red.

"Just making it red" seems like a straightforward approach...

Red Means "stop, explode, flammable, and hot." Green means "go, accept, Triangle, and recycle." Gamers love cliche's.

We had to use Red Barrels [People Can Fly blog]