Exterior paint jobs and decals can be displayed on your plane for aesthetic purposes but some can provide small camouflage bonuses depending on the map you're on and the camouflage pattern you have. In other words, picking a nice green to coat your plane will work great on a Summer map, but applying a Desert paint color won't help much in an Artic setting. Other things such as decals and nose jobs can be added for purely aesthetic purposes, and do not affect camouflage.
Purchasing Camouflage/Paint jobs
Exterior items can be bought for periods of 7 and 30 days with Silver or Permanently with Gold, however some patterns and decals can only be received as special bonuses (E.G. Beta Test participant, Commemorative token, etc.)