The term "blue signal" is actually referred to as green in English, which raises the question of why it is called "blue" in Japanese. This intriguing phenomenon reveals the close relationship between language and cognition.
In Japanese, the word "青" (ao) historically encompassed a broader concept of color that included "green." This can be seen in expressions like "青りんご" (green apple) and "青々とした芝生" (lush green grass). The first automatic three-color traffic signal in Japan was installed in Hibiya, Tokyo, in 1930, and it was initially referred to as "緑信号" (green signal). However, due to a legal revision in 1947, it officially became known as "青信号" (blue signal). The widespread use of the term "青" can be attributed to its original inclusion of green, its frequent appearance in newspapers, and the clarity of the combination of red, yellow, and blue.
According to the well-known Berlin and Kay study (1969), there is a universal order in the development of color terms. At the most basic level, only "white" and "black" are distinguished, and gradually terms like "red," "yellow," "green," and "blue" are added. The historical use of "青" in Japanese as a broad term that includes "green" has influenced its unique color perception.
These differences are not merely a matter of vocabulary. Recent studies using fMRI (functional magnetic resonance imaging) suggest that the classification of colors in one's native language may also affect visual processing in the brain. In other words, we view the world through the filter of "words," and this filter shapes the very way we perceive reality.