I'm a native Arabic speaker too.

First, khadra خضراء really is the femenine form of "green". The muscular form is akhdar اخضر

Second, it really is true that we use feminine forms when referring to multiple "it"s. You probably never consciously realized it, but think about it:

beautiful flowers ازهار جميلة

beautiful girls بنات جميلات

beautiful boys اولاد جميلون

Note how jameela is the singular feminine form of the adjective.

beautiful girl بنت جميلة

beautiful boy ولد جميل