I say tan. I nearly went with midnight blue for my Bronco but in the end decided to keep it dark green. I have a spice (kayline) top and think it looks real nice. The spice color looks great with the blue and sets it apart from all of the blue/black trucks out there.
On a side note, it's more difficult to find tan accessories such as seats, seat belts, floor mats, etc. (if you care about all that stuff matching).
-D