I've seen vehicles with decals saying PlanetSide Beta and others. How do I put one of these decals on my vehicle? And how do I get some more?
To apply them, open the pause menu, go to the vehicle tab, select the vehicle you want to customize, and choose a loadout:
Then pick "Apperance":
Then go to the decal tab, pick the decal you want, return to the game, and spawn a vehicle with that loadout.
That will take to you to this screen where you can filter to decals and unlock ones that you want for Station Cash:
Alternatively, you can buy them buy just going to the store tab and picking "Vehicle Gear":