Red means "stop," green means "go," and yellow means "hurry up and make that damn light." Why those colors, though? Why not blue, purple, and brown? I have to admit that aside from a hunch that it had to do with wavelengths, I had no idea myself, so I decided to look into it.
The answer, as it turns out, is a little convoluted, but makes sense. The earliest traffic signals were designed for trains, not cars. They were red and green, gas-powered, and more than a little dangerous in the event of a leak.