For each unlocked plane you are able to unlock a different skin for it. Aside from costing credits for the planes, you will also need to beat a challenge for that specific plane to unlock the extra skin.
Here's a pretty easy combo of plane price and challenge difficulty:
Amass 7,800 credits and then head over to the Hangar and buy the USSR IL-2. It's challenge is to kill 30 ground units in versus. Let's set it up:
Versus - Create Session - Kuban Battlefront Domination.
Unless you have another player with you make sure it is not private.
While in the lobby press to switch sides so you are able to play as the USSR with your starting plane now being the IL-2 since you just bought it.
During the mission you'll fly over the airfield. Make sure you don't get shot down (goes without saying) and destroy as many ground targets as you can. You can kill "X" number and then restart the level as the game will track the kills for you. After you complete the challenge you will be notified about the new skin and will unlock the achievement.