I think the brown and black smart cases look good on both iPad Airs. I'd do black Smart Case on the white iPad, and brown Smart Case on the black iPad. Both combinations I think pretty slick, but you could easily switch them.
Red looks OK, I'm just not a fan of the brighter colored cases. I think Beige would look OK, but I haven't seen a beige one in person yet. Looks kind of pink from the screenshots, which would be OK if you're into pink.