The object of these puzzles is to fill a colored object with only one color in as less turns as possible. The object can be a rectangle but also a 3-d figure.
Actually it doesn’t qualify as puzzle. You will always succeed in filling the object. The goal is to reach the target within x moves. To calculate the absolute minimum of moves all possibilities have to be tried. But the game is so addicting that I mention it here anyway. It gives some kind of zen feeling when playing the game.
Example of Floodfill puzzles are:
You can also read my blog about floodfill puzzles <link to blog>