Midnight looks terrible and will gunk up instantly. Not a fan of the blue tone at all because it doesn't look like an intentional royal dark blue, instead it looks like it can't decide if it's black or blue -- akin to cheap black dye. Given how thin and small the Air is I think Silver is the best way to go, very classic and timeless. I like Space Grey on my 16" because it makes your eye think the footprint is smaller.
That's the way I'll probably go with all future Apple devices: Spacey Grey on my 16" and iPhone, Silver for everything else (iPad, Apple Watch, Air, etc.)