1The color blue; azure.
彼のお気に入りの色は青だ。
His favorite color is blue.
この服は青と白、どちらがいいと思う？
Do you think blue or white looks better for this outfit?
壁を青に塗り替えた。
We repainted the wall blue.
海の青がとても鮮やかだった。
The blue of the sea was very vivid.
2Green — a greenish or unripe color. In compounds and contexts like plants, fruits, and traffic lights it can mean 'green'.
青リンゴはまだ酸っぱくて硬い。
The green apple is still sour and hard.
青ネギを小口切りにしてラーメンにのせた。
I sliced the green onion and put it on the ramen.
青野菜は栄養が豊富だ。
Green vegetables are nutritious.
3The green light in traffic signals (the 'go' signal); a casual abbreviation of 青信号.
信号が青になったので横断歩道を渡った。
The signal turned green, so I crossed the crosswalk.
青信号で車が次々と進んだ。
Cars went through one after another at the green light.
4A horse coat color called 'blue' — a dark, blackish coat color (specialized term).
その競走馬は青毛で、とても力強く走った。
That racehorse had a blue (dark) coat and ran very powerfully.
5In hanafuda (Japanese playing cards), the blue 5-point card (specialized term).
花札で青の5点札を引いたら、点数が高い。
If you draw the blue 5-point card in hanafuda, it scores high.
6As a prefix, it can mean immature, unripe, or young (e.g., 青二才, 青少年).
若い頃は青二才で失敗も多かった。
When I was young and inexperienced, I made many mistakes.
青少年の健全育成が課題になっている。
The healthy development of youth is becoming a key issue.
彼女は青春時代の思い出を大切にしている。
She treasures the memories of her youth.