I don't think it's really "legal" to pick and choose what rules from the Green Book you should be following. If you have more than 2 NPC sororities on your campus and have a college panhellenic, you should be following the Green Book. Also, if you're a collegiate chapter that doesn't have a Green Book, I would advise contacting your national or international HQs immediately and acquiring one
If you're in Canada, I think it's a different color though