Power Armor skins are buyable skins for your appearance in the game. You can find and purchase these skins in the character menu
under 'Power Suit Skins
You need atleast 4000 Coins
to buy the four cheapest skins for the achievement. (You have one skin already so you need only to buy 4 skins)
Coins can you obtain through (daily / Weekly / replay) missions or open coin chests in the world.