Four-Player Chess/Common openings/1. h3/1...d6
Sicilian Defence, 1...d6
Blue blocks the dark-squared a9-i1 diagonal with a pawn, giving him the option to play the move c8 next, if Red does not take the pawn with the bishop.
Yellow can choose between opening on the dark squares, like Red, or doing the opposite and opening on the light squares. The former is the classical way of playing and may lead to a symmetrical opening that is generally known as the "Classical opening", in which Red and Yellow develop their queens to j4 and e11, respectively, and Blue and Green develop their queens to b7 and m8. The latter is more modern and aims to coordinate threats along the light- and dark-squared diagonals. The most common moves for Yellow are g12 and h12 and somewhat less common is h11, pushing the queen's pawn two squares.
This classical move has regained popularity recently.
The idea of this move is to do the opposite of Red and open on the light squares to coordinate better with Red and create threats along both the dark- and light-squared diagonals. The follow-up is usually Qh13, potentially creating a mating threat on Blue together with the Red queen, if she can attack b9 or b7 later on, for instance with the manoeuvre Qg1-k5-g9. Yellow can also target Green with a move like Bg12, attacking Green's natural weakness on m6. This plan can be very dangerous, but there are sufficient defensive resources available to Blue and Green. In the long term the bishop can be slightly misplaced on g12, but usually it will either be sacrificed or moved to h11, taking control over the long diagonal.
This move has an idea similar to h12, but pushing the pawn one square further gives Yellow another option to develop the queen to h12. It does take away the option to play Bg12 and target Green's weakness on m6.