S1E20 : Green Isn't Your Color

When Rarity meets a famous fashion photographer, she realizes that this is her chance to promote her newest dresses, but her idea backfires when the pony is more interested in Fluttershy, whom Rarity chose to model her dresses. Both tell Twilight they aren't happy about this but don't want the other to know, and she is faced with a dilemma of keeping their secrets.