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Maybe dragons can't really distinguish between shades of purple.

Well, to be fair, Twilight and Starlight look similar, if Ember has not seen Twilight or Starlight frequently, she can easily confuse them. Their colors are similar, cutie marks have a star in them (though Starlight also has some green smoke or something coming from it, but maybe Ember though it was rude to stare at the cutie marks for too long). There is one problem though - Twilight has wings. Either Ember is really inattentive or she thought it was normal for a pony to lose and then grow back wings multiple times per day.