Microsoft Flight Simulator allows you to change the livery of your airplane, giving it a new appearance. While there are no liveries in the game yet, the first batch is due to arrive as a free add-on on August 19. There will, eventually, be plenty of different liveries in the game as developers Asobo and various third parties will end up filling the Marketplace with different options.
To change your livery, you need to open the World Map and then click on your aircraft in the top left corner. Select the aircraft you wish to fly, and you can then click on the Liveries tab. This will load up pictures of all the available liveries for that particular aircraft.
There have been no announcements yet about when new liveries will be added to the game, or how much they might cost, outside of the free pack that will be available from August 19 to September 30, so make sure you download them when they become available.