Poor Derpy. Behind her is a drooling Celestia out to take her cake.... ANNND
Behind Celestia (unseen but only by my imagination) is a drooling Luna about to take her..... um.. wish her a happy birthday. Ya!
Looking at the picture I thought it was Celestia but one part of her hair that is colored is dark blueish-purple. So I though, well maybe it was Luna. Then I say you said Sun Princess so ya, it's Celestia. Though that explains my over active imagination.