青

1. Origins, structure, and basic meaning

• Structure: 青(あお) is a noun for the color blue.
• Basic usage: It names the color of the sea or the state of a traffic signal.

2. Common pitfalls and similar expressions

• Noun and adjective
  • In 海の青, the color itself is a noun; in 青い海, 青い modifies the sea's color.
• The name of a traffic-light color
  • The Japanese expression 信号が青 corresponds to a green light in English.