There are many alternative paint jobs for every plane in the game (F-18 Super Hornet, not included); None of them are unlockable in any way, shape or form.
How come?
It's simple, in a marvelous technical achievement for the time, every enemy NPC uses the same polygon models as the player; Given that's the case, their textures should fit our own.
...and they do; They're the same resolution and just as detailed.