The yellow traffic light
In 1920 roads were becoming more crowded and more dangerous, so Detroit police officer William Potts converted railroad signal lights into the first traffic light. Railroad lights were initially white, green, and red, but in the early 1900s, yellow replaced white because it was deemed more visible.
The yellow pages
The business phone book owes its distinctive colour to a printer in Cheyenne, Wyoming, which in 1883 ran out of white paper while printing one of the first ever phone directories (the phone had only been patented a few years earlier). So the printer finished the job on yellow paper, and it caught on.
The yellow taxicab
In 1915 Chicago businessman John Hertz founded the Yellow Cab Company. He commissioned a university study to “scientifically ascertain which colour would stand out strongest at a distance.” The winner, of course, was yellow. But Hertz wasn’t the first: In 1909 Albert Rockwell operated a fleet of yellow cabs in New York City. He wasn’t as scientific, though – he chose yellow because it was his wife’s favourite colour.