While most puzzles of this type require one color (usually white) to be orthogonally contiguous, this requires both colors to be so. Hence the name.
Puzzle and Goal
An unsolved puzzle consists of a square grid. Some cells contain a white or black circle.
The goal is to fill all cells with either a white or a black circle.
The solved grid must satisfy the following conditions:
All white cells, and all black cells, must form an orthogonally contiguous region.
All 2x2 areas must contain at least one circle of each color.