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The flowers chosen for the names of two of the three major female characters in The Great Gatsby do, indeed, carry symbolic meanings.
A daisy is usually a white flower, with white being the color associated with innocence and angelic purity. Daisy Buchanan is not an angel in reality, but Gatsby never truly accepts that she could be anything other than the ideal he envisions in his dreams.
Daisy and Jordan lay upon an enormous couch, like silver idols weighing down their own white dresses against the singing breeze of the fans.
The flower of the myrtle, on the other hand, varies in the red-blue-purple range. Red is the color of passion; blue is a cool color frequently associated with calming effects. The first time Nick meets Myrtle Wilson, she is wearing a blue dress which doesn't disguise the attraction she and Tom feel toward each other.
Her face, above a spotted dress of dark blue crepe-de-chine, contained no facet or gleam of beauty...She smiled slowly shook hands with Tom, looking him flush in the eye.
Later, red is the color of blood staining Myrtle's dress after the accident.