Ok, do you know the part where Elle is buying a dress at the shop?? Well, she is trying on a BLUE dress and looks in the mirror and tells her friends "this is the one". Well later that night, when she is getting ready to be picked up by Warner to go on her date, She has on a PINK dress.
It is because before Ellie says that the blue dress "is the one" her friends tell her that she should go with the pink dress because it is her signiture color. So maybe she changed her mind and went with what her friends said, since it did not show her paying for any clothing.
Sometimes..not always..but sometimes, they actually have dresses available in more than one colour. It's entirly possible she tried it on, liked it, then bought it in her "signature colour".