A puzzle on a street of wooden houses. Each facade is made of boards, and the owner has drawn a design that gives the colour of every board. The roller paints a whole run at once, from an edge or a window to the next window or edge, and new paint covers the old. A stroke is one movement: drag your finger or mouse across the boards along a row or column, and the roller runs the same way.
It pays to read the design backwards. The last stroke is always a run that is a single colour in the design, and once you set it aside, the stroke before it comes into view. The leaderboard counts strokes, so the real skill is seeing which boards a later crossing stroke will cover anyway.