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“More derps. My guess is that while the Princesses almost never take their bracers & crowns off, they still change their attires regularly.  
Celie’s outfit and primary weapons might change over time, as she lived through a few different eras. She usually summons something like a magic shield for protection and neutralizing spells, being the guardian of Equestria and all, unless she goes all out. When she does go all out, she switches to the magic greatsword. She is kinda like a knight. Because she does such a good job of suppressing her evil side for long time, it doesn’t really have a form.  
Luna, on the other hand, simply summons the armguards and leg guards. Like Fluttershy, she is close to the animals, so she refuses to use weapons. As Nightmare Moon, her physical strength rivals Celestia’s, and her flesh darkens as if it is made of darkness. She can easily turn herself into mist or shadow to avoid and undo any physical harm. And she fights like a predator, relying on feral instincts. Her final form tones down the brute strength and the feral rage in favor of better speed, power focus and strategic thinking. I guess her final form “floats like a fly, sting like a bee.””