So, it turns out what this person wears to a ball is a fabulous 1960s cocktail frock with matching evening coat, in white, pale gold and silver. It's gorgeous, and it's for grown-ups!
Miraculously, it also fits like it was made for me and is incredibly flattering as well.
The ball is in two weeks, so I'll try to remember to post photos.
Off to do make up research. This is fun.