In the world of signals, it's an undisputed fact that red means stop. From red traffic lights to "prohibited" signs to, of course, the stop sign itself, red grabs your attention and sends a message to stop. But you probably didn't know that that red stop sign has only been the standard for about sixty years. Before then, the octagonal traffic sign was yellow, with the word STOP in black letters. It wasn't until 1954 that the stop sign became the bright red color, adorned with white letters, that we know today.
In the early 20th century, stop signs actually weren't any specific color or even shape. Understandably, the lack of standardization confused drivers, so the American Association of State Highway Officials convened in 1922 to select a standard design. This is how the octagon shape came to be. The AASHO wanted to choose a shape that even drivers coming in the other direction would recognize, so that they would know that the oncoming traffic had a stop sign. (Learn more about why traffic signs are different shapes.) They chose a yellow design with black letters, figuring the colors would grab drivers' attention.
Yellow wasn't their first choice, however. They actually did consider making stop signs red, since red already meant "stop" on electric traffic lights, which had been invented in 1912. (Here's why traffic lights are red, yellow, and green.) The problem with that was that, back then, there was no red dye that wouldn't fade over time.
By 1954, though, sign makers began using a fade-resistant porcelain enamel. The red color fading was no longer a problem. That year, the Joint Committee on Uniform Traffic Control Devices declared that, henceforth, stop signs would be red with white lettering.