Hello, I’m Kodak.
If you visit Japan, you may hear someone look at a clearly green traffic light and say:
信号が青になったよ。
Shingō ga ao ni natta yo.
The light turned green.
The traffic light is normally green or blue-green, but Japanese calls it 青信号 (ao shingō) because the traditional range of 青 (ao) included some colors that modern English speakers classify as green.
Japan’s “go” traffic light is called 青信号 (ao shingō), literally “blue signal,” even though its visible color is generally green or blue-green.
This is because 青 (ao) traditionally covered a broader blue-to-green range, and 青信号 became the established—and legally used—Japanese term.
Modern Japanese still has a normal word for green: 緑 (midori).
This article will explain:
② the difference between 青 (ao) and 緑 (midori)
③ which green objects are still described with ao
④ when you must use midori instead
⑤ how to say blue-green, bluish green, and greenish blue in Japanese
⑥ natural traffic-light example sentences and common beginner mistakes
Why Are Green Lights Called Blue in Japan?
The Japanese word for a green traffic signal is:
Hiragana: あおしんごう
Romaji: ao shingō
Literal translation: blue signal
Natural English translation: green light
There are three main reasons for this name.
Reason 2: 青信号 became a fixed, conventional expression in Japanese.
Reason 3: Japan’s traffic regulations officially use the wording 青色の灯火—“an ao-colored light.”
Reason 1: The Traditional Meaning of Ao Included Green
In modern beginner textbooks, the basic translation is simple:
緑 (midori) = green
However, the older and broader Japanese concept of 青 included not only blue but also greenish colors, especially the fresh colors of plants.
Japanese dictionaries still record 青 as covering shades ranging from indigo and blue into greenish colors. This older range survives in established words such as 青葉 (green leaves), 青りんご (green apple), and 青信号 (green traffic light).
English and Japanese can divide the visible color spectrum into different named categories while their speakers still see the same color differences.
Modern English usually separates:
blue | green
Traditional Japanese ao could cover a broader area:
blue | blue-green | fresh plant green
Reason 2: 青信号 Became a Fixed Japanese Expression
Once a phrase becomes conventional, speakers do not reconstruct its meaning from each individual word every time they use it.
Japanese speakers learn 青信号 as the ordinary name of the signal that allows traffic to proceed, just as English speakers learn green light.
This is similar to expressions in many languages whose literal wording does not perfectly match a modern scientific color label.
Literal wording: blue signal
Actual meaning: the green “go” light
青信号が出る
Literal wording: a blue signal appears
Figurative meaning: to receive approval or the go-ahead
新しい計画に青信号が出た。
Atarashii keikaku ni ao shingō ga deta.
The new plan got the green light.
Reason 3: Japanese Traffic Law Uses the Word Ao
The terminology is not limited to casual conversation. Japan’s Road Traffic Act Enforcement Order refers to the “go” indication as 青色の灯火 (ao-iro no tōka), literally an “ao-colored light.”
In natural English: green light
Not: blue light
Is Japan’s “Blue” Traffic Light Actually Blue or Green?
To most observers, the illuminated “go” signal is green or blue-green rather than the blue of a clear sky.
Japanese dictionaries describe 青信号 as a blue, green, or blue-green traffic signal. The exact appearance can also vary with the signal equipment, lighting conditions, camera, screen, and the observer.
If you are discussing its exact physical hue, you can say 緑色に見える (“it looks green”) or 青緑色だ (“it is blue-green”).
日本では、緑色に見える信号を青信号と呼びます。
Nihon de wa, midori-iro ni mieru shingō o ao shingō to yobimasu.
In Japan, a signal that looks green is called an ao shingō.
この信号は少し青緑色に見える。
Kono shingō wa sukoshi aomidori-iro ni mieru.
This traffic light looks slightly blue-green.
Ao vs Midori: What Is the Difference?
For present-day everyday Japanese, the basic distinction is:
blue; sometimes green in traditional or fixed expressions
緑 (midori)
green as a modern color category; also greenery or vegetation
Use ao for blue—and for certain established expressions where Japanese traditionally uses ao.
Use Ao for Ordinary Blue Objects
青い空
aoi sora
a blue sky
青いシャツ
aoi shatsu
a blue shirt
青い海
aoi umi
a blue sea
Use Midori for Ordinary Green Objects
緑のシャツ
midori no shatsu
a green shirt
緑の車
midori no kuruma
a green car
緑のペン
midori no pen
a green pen
青いシャツ = blue shirt
緑のシャツ = green shirt
Why Does Midori Mean Green?
緑 (midori) is the normal modern Japanese word for green. Dictionaries explain that the word originally referred to young shoots or fresh buds and later developed into a color name.
new buds and young plant growth
↓
the fresh color of those plants
↓
green as a color name
Green Things That Japanese Often Calls Ao
The safest approach is to learn these as vocabulary items rather than using ao freely for every green object.
1. 青信号 (Ao Shingō): Green Traffic Light
青信号になったら渡ってください。
Ao shingō ni nattara watatte kudasai.
Please cross when the light turns green.
2. 青葉 (Aoba): Green Leaves / Fresh Leaves
木々の青葉がきれいです。
Kigi no aoba ga kirei desu.
The trees’ fresh green leaves are beautiful.
3. 青草 (Aokusa): Green Grass / Fresh Grass
馬が青草を食べている。
Uma ga aokusa o tabete iru.
The horse is eating fresh green grass.
4. 青りんご (Aoringo): Green Apple
青りんごのジュースを飲みました。
Aoringo no jūsu o nomimashita.
I drank green-apple juice.
5. 青菜 (Aona): Leafy Green Vegetables
青菜をさっとゆでます。
Aona o satto yudemasu.
Lightly boil the leafy greens.
6. 青虫 (Aomushi): Green Caterpillar
葉の上に青虫がいる。
Ha no ue ni aomushi ga iru.
There is a green caterpillar on the leaf.
When You Should Use Midori, Not Ao
For most ordinary green-colored objects, use 緑.
緑のかばん — a green bag
緑の目 — green eyes
緑の壁 — a green wall
緑色の絵の具 — green paint
緑茶 — green tea
The historical explanation helps you understand fixed words such as 青信号; it does not make ao a free replacement for midori.
For a new object you have never learned before:
・blue ⇒ 青
・green ⇒ 緑
Learn exceptions such as 青信号 and 青りんご as complete words.
Ao vs Midori in Similar Examples
青い車
a blue car
⇒ The car’s color is blue.
緑の車
a green car
⇒ The car’s color is green.
青信号
a green traffic light
⇒ A fixed conventional term.
緑色の信号に見える
It looks like a green-colored signal.
⇒ A description of the visible hue, not the conventional name.
青いりんご
a green/unripe apple
⇒ 青い can describe unripe fruit.
青りんご
a green apple / a green-apple variety or flavor
⇒ A familiar compound noun.
For example, まだ青い can mean “still immature.”
How to Say Blue-Green in Japanese
The direct Japanese word for blue-green is:
Hiragana: あおみどり
Romaji: aomidori
Meaning: blue-green; a color between blue and green
青緑色の海
aomidori-iro no umi
a blue-green sea
この石は青緑色です。
Kono ishi wa aomidori-iro desu.
This stone is blue-green.
Bluish Green
aomi ga katta midori
bluish green; green with a blue tinge
Greenish Blue
midori gakatta ao
greenish blue; blue with a green tinge
青みがかった緑 says the base color is green, but it has a bluish tint.
青みがかった緑 ⇒ mainly green, slightly blue
緑がかった青 ⇒ mainly blue, slightly green
Pronunciation: Ao, Midori, and Ao Shingō
Hiragana: あお
Romaji: ao
Morae: あ・お
Pronounce both vowels: “a-o,” not one English-style diphthong.
緑
Hiragana: みどり
Romaji: midori
Morae: み・ど・り
青信号
Hiragana: あおしんごう
Romaji: ao shingō
Morae: あ・お・し・ん・ご・う
Useful Japanese Phrases About Traffic Lights
赤信号 — red light / stop signal
黄色信号 or 黄信号 — yellow light
信号機 — traffic light / traffic signal device
横断歩道 — crosswalk
信号が変わる — the light changes
信号を渡る — cross at the traffic light
信号が青になるまで待ちましょう。
Shingō ga ao ni naru made machimashō.
Let’s wait until the light turns green.
青信号でも、車に気をつけてください。
Ao shingō demo, kuruma ni ki o tsukete kudasai.
Even when the light is green, please watch out for cars.
もうすぐ信号が赤に変わります。
Mō sugu shingō ga aka ni kawarimasu.
The light will turn red soon.
横断歩道を青信号で渡りました。
Ōdan hodō o ao shingō de watarimashita.
I crossed the crosswalk while the light was green.
A Beginner Conversation at a Japanese Crosswalk
Mada wataranai no?
Aren’t we crossing yet?
Mada aka shingō da yo.
The light is still red.
A, ao ni natta!
Oh, it turned green!
Un. Demo migi kara kuru kuruma ni ki o tsukete.
Yeah—but watch out for cars coming from the right.
Common Beginner Mistakes
Mistake 1: Translating 青信号 as “Blue Light”
This sounds as though the lamp actually became blue.
○ The traffic light turned green.
This is the natural English translation of 信号が青になった.
Mistake 2: Calling Every Green Object Ao
This normally means a blue bag.
○ 緑のかばん
a green bag
Mistake 3: Saying 緑信号 Instead of 青信号
緑信号 may be logically understandable, but it is not the standard everyday name. Use 青信号.
Mistake 4: Assuming Ao and Midori Never Overlap
They are distinct modern color words, but historical and fixed expressions create areas of overlap.
Then memorize a small set of established ao words—especially 青信号, 青葉, and 青りんご.
FAQ About Ao, Midori, and Japanese Green Lights
Why do Japanese people call a green light blue?
Japanese calls a green traffic signal 青信号 because the traditional range of 青 (ao) included greenish colors, the expression became conventional, and official traffic terminology uses 青.
Is a Japanese traffic light really blue?
The “go” signal normally appears green or blue-green rather than sky blue. However, its established Japanese name is 青信号.
What is the Japanese word for green?
The ordinary modern Japanese word for green is 緑 (midori). Use it for green clothes, cars, walls, pens, and most other ordinary green objects.
What is the difference between ao and midori?
Ao normally means blue in modern Japanese but remains in some fixed expressions for green things. Midori is the standard modern color word for green.
Does ao mean both blue and green?
It can, depending on the word and context. For a normal modern color description, ao means blue. In established expressions such as 青信号 and 青葉, it refers to something visually green.
How do you say blue-green in Japanese?
青緑 (aomidori) means blue-green. 青みがかった緑 means bluish green, while 緑がかった青 means greenish blue.
How do you say “the light turned green” in Japanese?
You can say 信号が青になった or, when the traffic light is obvious, simply 青になった.
Can I say midori shingō?
緑信号 (midori shingō) is not the standard everyday expression. Use 青信号 (ao shingō).
Does midori come from a word for plants?
Japanese dictionaries explain that midori originally referred to young shoots or fresh buds and later became a color name.
Are Japanese people unable to distinguish blue and green?
No. The issue is how colors have historically been categorized and named in Japanese, not an inability to see the visual difference.
Summary: Why Japan Calls a Green Light Ao
Its literal parts are “ao + signal,” but the natural English translation is “green light.”
● Traditional ao covered a broad blue-green range
This usage remains in words connected with traffic signals, leaves, grass, apples, and other fresh green things.
● Modern Japanese distinguishes ao and midori
青 = blue
緑 = green
● Do not use ao for every green object
A green shirt is 緑のシャツ, not normally 青いシャツ.
● Blue-green is 青緑
青みがかった緑 = bluish green
緑がかった青 = greenish blue
● The easiest beginner rule
Use ao for blue and midori for green, then learn traditional exceptions as complete vocabulary items.
Related Articles
What Does Ao Mean in Japanese?
References
- e-Gov Law Search: Road Traffic Act Enforcement Order — official use of the term 青色の灯火.
- Kotobank: 青 — dictionary definitions covering blue and greenish shades.
- Kotobank: 青信号 — definitions of the green or blue-green go signal.
- Kotobank: 緑 — origin in young shoots and development as a green color term.
- National Diet Library Collaborative Reference Database: Why Green Is Sometimes Called Ao.