You can change skins in the hangar just prior to entering the cockpit, along with fuel quantity, convergence and loadout.
Mind you, the skins were removed from the body of the game some time ago to allow user created skins to be added periodically as approved for mods off running. There's always new ones being added, and there's about a zillion to choose from, but you have to use the skins packs available
HERE or you'll only see the single default skin available for each plane.
I suggest downloading all of them, so if someone's using a 'fun' skin you'll be able to see it even if you aren't interested in using it yourself. Just download each and run the installer, it's automated so you don't have to fool with anything other than pointing it at your main ROF folder.
[MF] Gunloon