For each cell in the board, we define a critical mass. The critical mass is equal to the number of orthogonally adjacent cells. That would be 4 for usual cells, 3 for cells in the edge and 2 for cells in the corner.
All cells are initially empty. The Blue and the Green player take turns to place "bombs" of their corresponding colors. The Blue player can only place a (blue) bomb in an empty cell or a cell which already contains one or more blue bombs. When two or more bombs are placed in the same cell, they stack up.
When a cell is loaded with a number of bombs equal to its critical mass, the stack immediately explodes. As a result of the explosion, to each of the orthogonally adjacent cells, a bomb is added and the initial cell looses as many bombs as its critical mass. The explosions might result in overloading of an adjacent cell and the chain reaction of explosion continues until every cell is stable.
When a blue cell explodes and there are green cells around, the green cells are converted to blue and the other rules of explosions still follow. The same rule is applicable for other colors.
The winner is declared when the opposing player no longer has any bombs in play.
* Rule description modified from brilliant.org, however the game itself is a reimplementation of something I played on the Atari 8-bit back in the 80's.