And Paul's Bird Was Green

If you listen to John's all-emotion song, And Your Bird Can Sing [listen at:  www.youtube.com/watch?v=iC6D2N4nylg ], there are three more clues that Paul was headed to France and a possible romantic involvement with Sylvie Vartan.
John says the bird (girl) of the man he's singing about:  1.) can sing
                                                                                  2.) can swing
                                                                                  3.)  is green.
Green in this context doesn't mean romantically naive because the girl can swing.  So what does green mean? 
In the book, The American Language (1921) by American linguist H.L. Mencken, he is talking about English World War 1 Army slang and mentions that Frenchmen were called frogs by British soldiers.  Green.  Sylvie Vartan could sing, was married and was French and I think John's song was about John being abandoned by Paul if Paul went to France.