Well, given that her name is Twilight Sparkle, the six stars on her flank (which have 42 points by the way...hmmm), and her colors, it must have something to do with the evening.
My theory is that Luna, though she has power over the moon, can only bring out so few stars, and Twilight is meant to bring out a lot more. Buuuuut that's probably not true, it's just a guess.