class ItemValidator(object):

    def __init__(self, items):
        self.items = items

    def validate(self):
        if Item("Green") in items:
             raise MyGreenValidationError
        return True

>>> f = Foo()
>>> item = Item("Green")
>>> f.items.add(item)
>>> i = ItemValidator(items)
>>> if i.validate():
>>>      f.items.save() # or something