First you have to buy the plane, after unlocking you get one livery for free which is snow white. But for the others liveries, you have to unlock it by using golden planes. When you used your golden plane to unlock the type then you have to spend money on it to buy it.
Simpler explanation- Buy livery with golden planes. Then go to airport, click on fleet store, select that livery and buy it with money.
The same process if you want to unlock Iberia ATR right?
No its only for fleet planes. You cannot control any airline planes. When you enter the airport, you can see an option with a plane on it. Click it and choose what you want to buy. You cannot use any airlines in your fleet.
Check this topic, it might give you some information - Playing the game