[That greeting possibly isn't something she'd direct at Sebastian. Maybe? Either way I'm creating my own scenario: Sebastian also likes parties. But he likes them more when he gets to serve at them, and when he's been responsible in some measure for planning them, and when they aren't hosted by Lucifer.
He's here, either way, currently loading a plate with snacks to present to the young master (because of course Sebastian insisted that Ciel should attend. Ciel doesn't like parties! That's a perfect reason to insist), and raises his eyebrows in surprise slightly as he sees the dress. Then he smiles.]
If I may, ma'am, you look lovely. [In the dress. The dress Sebastian made.]