Japanese used to have a color word, ao, that spanned both green and blue. In the modern language, however, ao has come to be restricted mostly to blue shades, and green is usually expressed by the word midori (although even today ao can still refer to the green of freshness or unripeness—green apples, for instance, are called ao ringo). When the first traffic lights were imported from the United States and installed in Japan in the 1930s, they were just as green as anywhere else. Nevertheless, in common parlance the go light was dubbed ao shingoo, perhaps because the three primary colors on Japanese artists’ palettes are traditionally aka (red), kiiro (yellow), and ao. The label ao for a green light did not appear so out of the ordinary at first, because of the remaining associations of the word ao with greenness. But over time, the discrepancy between the green color and the dominant meaning of the word ao began to feel jarring. Nations with a weaker spine might have opted for the feeble solution of simply changing the official name of the go light to midori. Not so the Japanese. Rather than alter the name to fit reality, the Japanese government decreed in 1973 that reality should be altered to fit the name: henceforth, go lights would be a color that better corresponded to the dominant meaning of ao. Alas, it was impossible to change the color to real blue, because Japan is party to an international convention that ensures road signs have a measure of uniformity around the globe. The solution was thus to make the ao light as bluish as possible while still being officially green.
Guy Deutscher, Through the Language Glass: Why the World Looks Different in Other Languages