That's called a lime.
Unless it's an unripe lemon.
il limone è verde - can also translate to the lemon is unripe
Before they get yellow they are green.
In my language (afrikaans) we say fruit are green if they are unripe. Even if it is literally not the colour green. I imagine the same principle applies to Italian.
Come on....all clever explanations aside...a lime is green, a lemon is yellow. An unripe lemon is green yes, but then you have to say it is unripe... :-). In french a lime is called a green lemon so maybe the solution lies therein!