Color complex

The board has thirty-two light squares and thirty-two dark squares, and at many moments in a game one of those sets matters far more than the other. When that happens the position is said to have a colour complex — a weakness or strength concentrated on squares of a single colour. A player whose bishop and pawns conspire to control the dark squares while leaving the light squares undefended has a light-square weakness, and the opponent’s light-squared pieces become the most important on the board.

Colour complexes are created in three ways. The most common is a fianchetto in which the bishop is exchanged: the king now stands on a square whose defender has vanished, and every square around the king of the bishop’s old colour becomes a potential entry point. The second is a chain of pawn moves on one colour: e3, d4, f4 fixes White’s pawns on dark squares, meaning the light squares are now reachable only by pieces. The third is the loss of a single key piece — usually a bishop — that was holding one colour together.

The classical examples come from the Sicilian Dragon, in which the exchange of dark-squared bishops by sacrificing the exchange on c3 hands Black the dark-square complex around White’s king. Once those squares are in Black’s hands, every White pawn move on a light square only deepens the weakness, and even passive defence cannot save the king.

Colour complexes are evaluated holistically, not by counting squares. Three weak squares around the king matter more than ten weak squares in the centre. The presence or absence of the bishop of the relevant colour is usually decisive: with it, the complex is defensible; without it, the complex is often terminal.