United States Standard for the Colors of Signal Lights [pdf]
This NIST handbook (formerly NBS Handbook 95) establishes the United States standard for the colors of signal lights, defining color specifications and boundaries for red, yellow, green, blue, and white signal lights used in transportation and other applications.
Background
This is a 1973 U.S. government standard (NBS Handbook 95) that defined the precise chromaticity coordinates for red, yellow, green, blue, and white signal lights — used in traffic signals, aviation beacons, railway lights, and maritime navigation. Before this standard, different agencies used different color specifications, creating safety risks. The document emerged from Cold War-era efforts by the National Bureau of Standards (now NIST) and a joint U.S.-Canadian committee to harmonize signal colors across rail, road, air, and sea. It remains influential because modern LED traffic signals had to be designed to stay within the color boundaries established in this handbook, even though LED technology didn't exist when the standard was written.