Now everybody would say zielenio
I'd say that now every one would avoid using it at all. This is one of those examples that are used so seldom that most people are not sure what is the correct form.
"Zieleni" is the correct form, but people just dont 'feel' it, because they don't need to use such word in everyday speech. On the other hand, they do use vocative of words like kobieto, dziewczyno, etc. so when they see words like 'zieleń' they're trying to apply the same rules they're used to.
Therefore, monsters like 'zielenio' are born.