That's a tough choice since color is such a personal thing. I bought the beautiful fuschia cover for my 700 and couldn't be happier.
If you are feeling particularly royal, you should choose the color of royalty, purple. If you are feeling a little "racy" or passionate, choose the color of passion, red. If you are feeling conservative or academic, choose the colors of academia, beige and brown.
Whatever you choose, the covers are marvelous and a great purchase.
Good luck deciding.