You can change the appearance of the following buildings for free:
- Farm House
- Barn
- Fountain
For the Water Tower and Old Silo, changing their appearance will cost Decoration Coins. To change the appearance of any of the buildings, tap on it then tap the Brush icon. 
Select the style that you want to use by tapping on the Apply button. 
For the Farm House and Fountain, other styles will become available as you level up in the game.
For the Barn, Water Tower and Old Silo, unlock other styles by upgrading these buildings.
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FarmVille Game GUIDE