class ItemValidator(object):

    def __init__(self, items, item=None):
        self.items = items
        self.item = item

    def validate(self):
        if self.item:
             if self.item.color = "Green" and Item("Red") in self.items: # or something like that
                   raise MyGreenValidationError
        return True

>>> f = Foo()
>>> item = Item("Green")
>>> i = ItemValidator(f.items, item=item)
>>> if i.validate():
>>>     f.items.add(item)