AFAICT, once you've purchased a roof tile you leave it (cost) face up until you actually use its bonus, which can be at any time depending on the colors printed on the tile.
That is correct. Don't forget to take your VPs (if any) and don't forget to flip the tile when you use it. It's a one time bonus. You can only use them once in the whole game. Except for the player who gets a special tile in the last round. With this one you can flip another one back and use it twice...