Seeing the bright and colourful rainbow pone in that dark attire and demeaner gets him right in the funny bone. She is playfully doing it to spite the poor lad because she too wanted to go as Starswirl the bearded as well for nightmare night just for the irony of her being an earth pony, but being the big magic nerd he is he beat her to it and now she wants to playfully sulk and guilt trip him, mares eh?