She changed into the black dress late in the afternoon for dinner.
From Vanity Fair:
But Trump changed into something more formal later on in the day, switching into a long, lace off-the-shoulder dress and a pair of strappy black heels. The group posed for a final photo together before going to the Royal Castle for dinner.
I suppose calling her the First Lady is above their intelligence level...
Calling her “Trump” is rude.