Is it true that copper turns into a dull green colour when it is exposed to the air?

The above statement is true.

A green layer forms on the floor of a copper vessel while its miles are exposed to damp air for an extended length of time. Copper corrodes through oxidation, which happens while it combines with oxygen inside the air to form oxide. Copper carbonate is formed while oxide reacts with CO

2

, giving it a green tint.