Fiver

The goal is to convert all of the White pieces on the board to Black pieces. Clicking on a piece with the mouse will change it from White to Black, and vice-versa. It will also change the pieces above, below, and to the left and right of the selected piece.

The Moves counter will turn red to indicate the minimum number of moves necessary to win, for the selected board size. Beyond the minimum number of moves, the Moves counter will turn gray.

(Thanks to Gabriel L. Schmidt for the 4-move solution to the 4x4 board.)