"The lemon is green."
Translation:O limão é verde.
here, there is just limão. The "standard" one, that one that people automatically understand by limão is the green one...The yellow is called limão siciliano and there are other kinds like limão-galego, limão-rosa..etc..
Strange English sentence. I understand that in Brazil they call both lemons and limes limao, but they are different words in English; lemons are yellow, limes are green.
As a matter of fact, unripe lemons are green* so without further context it's impossible to say if this sentence is about a lemon or a lime.
* Which can be a problem because although they might look similar they taste very different.