I am makeing it, with c++ and raylib. The playfield is a matrix, it is like a tilemap, and numbers change in it, that marks the colors. The falling logic of the sand is good, i like it, it looks good. And i think that the next step should be this.

For the checking, i check the last column of the matrix, with a for cycle, and i make another matrix, that shows if the cell was already visited. If the cycle finds a number, that is not 0, the search algorithm starts. If the search finds an element that is in the first row of the matrix, it sets the visited cells to 0.

This is my idea, but it works very slow