How can it be the lipstick? And yet, it is.
The lipstick means that "Violet" is in the midst of getting ready for some occasion--in the midst of the dressing-up process. The pose of displaying her panties, as she is, would inspire a very different narrative without the idea that someone has interrupted her dressing-up.
Together with the radiant smile, it suggested to me that Violet has been a very good girl. She's happy to show off the lovely lacy panties.
In turn, the way my mind works, at least, I'm forced to think of an occasion on which Violet was not such a good girl, and of what the consequences might have been.
Finally, because her smile radiates so much warmth, I think she must be anticipating something wonderful: by now you must know that as far as I'm concerned that wonderful thing could only be a spanking.