解释全文A. What’s a White lie Mary did not understand such sente

解释全文
A. What’s a White lie
Mary did not understand such sentences as “She is blue today,” “ He has a green thumb,” “He has told a little white lie” and so on. And she went to her teacher for help.
Mary: Mrs Smith, there is a colour in each of these sentences. What do they mean?
Mrs Smith: In everyday English, Mary, blue sometimes means sad. Yellow… afraid. A person with a green thumb grows plants well. And a white lie is not a bad one.
Mary: Would you give me an example for “a white lie?
Mrs Smith: Certainly, now I give you some cakes. In fact you don’t like it, but you won’t say it. Instead, you say, “No, thanks, I’m not hungry.” That’s a white lie.