objects = {
    'A': {'Red', 'Ford'},
    'B': {'Red', 'Ford'},
    'C': {'Blue', 'Ford'},
    'D': {'Green', 'Volvo'},
    }

attributes = set()
for atts in objects.values():
    attributes |= atts
attribute2objects = {att: set(k for k, v in objects.items() if att in v) for att in attributes}
diffby1 = set()
for att1 in attribute2objects:
    for att2 in attribute2objects:
        if att1 != att2:
            ob1 = attribute2objects[att1]
            ob2 = attribute2objects[att2]
            if len(ob2) > len(ob2):
                ob1, ob2 = ob2, ob1
                att1, att2 = att2, att1
            if len(ob1 - ob2) == 1 and len(ob2 - ob1) == 0:
                diffby1.add((att1, att2))
print(sorted(diffby1))