Blue; of the color blue (like the sky or sea).
今日は空がとても青いので散歩に出かけよう。
The sky is very blue today, so let's go for a walk.
彼女は青いドレスを着てパーティーに来た。
She came to the party wearing a blue dress.
青い海が広がる景色に感動した。
I was moved by the view of the wide blue sea.
信号が青に変わったから渡りましょう。
The signal turned green/blue, so let's cross. (Japanese uses 青 for the traffic signal)
Green (archaic or specific usage): in some contexts, especially for plants, fruit, or traffic signals, 'ao(i)' can refer to green.
熟していないリンゴはまだ青い。
An unripe apple is still green.
日本では信号の「青」は実際には緑色のことを指す場合がある。
In Japan, the traffic signal called 'ao' sometimes actually refers to a green color.
Pale; having a bluish or ashen complexion (e.g., from illness or shock).
急に寒くて彼は青い顔をして震えていた。
It suddenly got cold and he was trembling with a pale/blue face.
心配で母は青い顔をして病院に来た。
Worried, my mother came to the hospital looking pale.
Unripe; inexperienced. Refers to fruit that is not ripe or a person who lacks experience.
その果物はまだ青くて酸っぱいから、後で食べよう。
That fruit is still unripe and sour, so let's eat it later.
彼は仕事は熱心だが青いところがあるので、もう少し経験が必要だ。
He's enthusiastic about his work but inexperienced, so he needs a bit more experience.
青い果実を収穫してしまうと甘みが足りない。
If you harvest unripe fruit, it won't be sweet enough.